I recently bought a quadplex and my insurance company requires me to replace all 4 water heater (15 yr+ require replacement, standard in my area). I found some 40 gallon at home depot for about $350 each. 

I got quote to replace water heater at $500-1000. I am considering replacing them my self. I live/househack one and another unit is vacant. I might hire out the two with tenants. But, how hard is it to repace a water heater? Any advice? I am looking at an electric water heater, closet heater, first floor.

I only owned this place for a month now and I'm trying to save while I build reserves. 

Pretty easy.

For electric:

Turn off breaker, make sure power is off at water heater.

Hook a garden hose to drain, open valve and let heater be draining.

Disconnect power, 2 wires and a ground.

Disconnect water lines.

See if HD will give you a discount since your buying 4 of these.

Once tank is drained removed tank with help from a friend or a dolly.

Reverse the process.

Don't turn power back on till tank is full of water.

Check with your utility company to see if there are any rebates or incentives.

I replaced the one at my VRBO lake house with a new water heater and my utility company sent me a check for $300. I just had to fill out a form and submit the receipt.

I've personally never hired out a job like that. They are super easy to swap out if you know how to do basic plumbing / electric (assuming there isn't and other issue, like they're located somewhere hard to reach). 

Probably the most difficult part of the job is dragging the old one out of the building and disposing of it. 

It's about the easiest thing you can DIY. You don't even have to learn to solder if you don't want to. Just use SharkBite fittings.

Watch some videos on youtube and your now qualified .

@Johnny Dang, I think your warranties and insurance company may require that they be installed by licensed professionals. It's worth confirming before you void something.

Also, it's worth considering tankless models. They are competitively priced, last longer, will never rupture and flood your property. 

How Valuable is your time? 

It will take about 3 hours when you factor in going to lowes to buy one. Replacing it. Then disposing of the old one.

They hard cost of a hotter tank is about $550. So if you get a quote to replace it for $800. Your only paying $250 for someone else to replace it / $83 per hour

But maybe your time is only worth $30 per hour. In that case, do it yourself and save some money.

@Johnny Dang  As mentioned above, make sure your insurance company allows you to do it.  Also that you're up to the latest codes.

Here in VT, they recently passed a new code saying all rental property water heaters now need a "mixer value" that prevents the water from being too hot in the HWH.  Without it and someone gets scalded, big problems.  They typically don't come with HWH and plumbing companies install it as part of the installation process.

Take a look at how the existing water heater is connected to the pipes.  In super perfect happy world, the plumber installed unions on both pipes right above the heater, so you can just unscrew the unions, pull the heater out a few inches, unscrew the pipes off of the top of the heater, and you're done.  You can use the same pipes on the new heater, as long as the distance between the two pipes on top of the new heater is the same as the old one, and the new heater is the same height as the old one.  If the distance or height is different, you'll have to do more work - shorten the pipes between the union and the water heater if the new heater is taller, lengthen them if the new heater is shorter, or remove the other half of the union and install a flexible pipe (see below).

Sometimes, instead of unions, there are flexible pipes from the rigid pipe to the water heater - either copper tubing (thinner than normal copper pipe, so you can bend it a little), or flexible hoses, sort of like what you hook up a toilet or sink with.  Copper tubing can be re-used if it lines up with the pipes on the new heater.  Flexible hoses should be replaced when you put in a new heater.  The flexible hoses only come in certain lengths.  They can be curved or even looped, but they can't be bent too sharply or they will kink.  Sometimes it takes a little experimentation to find the right length.  On my heater at home, the pipes from the house come straight down vertically from the ceiling.  What turned out to be right was for one flexible hose to run straight up from the heater to the vertical pipe, and for the other flexible hose to run up from the heater, turn 90 degrees, and connect to a 90 degree elbow that is on the vertical pipe.

Check with the local building code to see if you need an expansion tank on the new water heaters.  When the heater is on, the water expands a little.  Some of that raises the pressure in the hot-water pipes in your house, and some of that raises the pressure in the cold-water pipes and back out to the city water main.  Previously, letting it expand into the cold water pipes was considered OK, but now it's not considered OK in some places.  The expansion tank is a small metal tank, about 8" around and 8" high, that you plumb into the cold water line going to the water heater.  A lot of times you can put it on the wall above the heater.  Home Depot will have them in stock.

On the electric, if the cable is long enough, I would cut off the stripped (bare) ends of the wire, and strip the wire again, to get a "fresh" wire to connect to the new heater.  Usually the cable comes into the top of the heater at about the same place on all of them, but sometimes the cable turns out to be way too short for the new heater.  You can put in a junction box on the wall, run the old cable into the junction box, and then run new cable of the same gauge from the junction box to the water heater.  Use wire nuts inside the junction box to join the new and old cables.

You should also buy a new temperature and pressure relief valve (safety valve) for each new heater, if it doesn't come with one in the box.  On the old heater, you will see that this valve has a pipe that runs from the valve, down the side of the heater, and just ends a few inches above the floor.  You can usually re-use that pipe on the new heater.  If not, Home Depot sells pipes for this already made up, or you can get the fittings and make one.

If you have enough room, and a place for the water to go, you might consider installing a drain pan under the new heater.  This is just a round metal pan, about 2" or 3" deep and bigger around than the heater, with a hole in one side for a drain pipe.  It goes on the floor, and then the heater sits in the pan.  You then run a PVC pipe from the pan to a nearby floor drain, or to outside the house.  This pan only does something if the heater ever leaks; it will catch most of the water and direct it away from your carpet/flooring into the drain or the yard.

Before you scrap the old ones, check the parts list on the new heaters, or look at both the new and old heaters, to see if any of the parts are the same - on an electric, they often are.  If so, you might take the thermostats, heating elements, and drain valve off of one of the old heaters, and put them in your stash.  These parts are readily available new at the hardware store, and normally, you would want to replace a failed part with a new part.  But if you ever have a tenant with no hot water on Christmas Eve or something, the old parts might save your butt.  :)

Do the one in your unit or the one in the vacant unit first.  It will take you a while and probably some extra trips to the store.  Once you've done that, swapping the ones in the occupied units will go faster.

If you have a truck, you can haul the old heaters to the scrap yard and make a buck or two on them.  If not, maybe stash them in the vacant unit until you have all 4 replaced.  Then, put all 4 old ones out at the end of your driveway some morning.  If they don't disappear by lunch, put an "old water heaters for scrap" ad in the "free" section of your local Craigslist, and they will probably disappear by the next day.