Something to bear in mind is that contractors are not all created equally. Most investors (Heck, most PEOPLE) don't realize that a truly good, dedicated contractor is going to have some pretty major overhead expenses.
A question to ask - is the guy who is 4 times cheaper paying for insurance? Licensing? Dedicated trucks/vans? Communications? Job clocks? Shop space? Back-end scheduling software? Office help?
In other words - things that should theoretically make your job go easier, smoother, and faster than a 'buck with a truck'
Most contractors make lousy salesmen who think that customers don't want to hear about their real value-add proposition. That can make it very hard to differentiate between a guy running a legitimate business with legitimate backend, and a guy whose sole concern is his gas and cell phone.
This can go the other way too, though. The guy who is 4 times more expensive may be that much because he works one job at a time with no back end and no help, so to make up for his lack of sales he has to make sure that every win is a hit out of the park.
Meantime, the guy who is 4 times cheaper may have 60 guys working for him with 12 dedicated vans and a full time office staff and a metric ton of automation on his side. So he's selling cheap and making up for it with economy of scale while running just enough manpower to service 20 or 30 jobs at the same time... so while your schedule might suffer, you'll get it cheaper and more reliably done [eventually]
Speaking as a contractor, it's hard as crap to make a good balance between sales/manpower/backend/cost. Economy of scale is a very really thing in the contracting world, but it's also easy to get out of whack and end up with a long list of PO'd customers because... oops... I went out and outsold our capacity and we're simply undermanned.
Of course, the most likely thing here is that the less expensive guy is an unreliable flake and the more expensive guy is just... more expensive.
This is a really round-about way of backing up what J just said about getting 10 more bids. To dovetail onto that - the ivnestor/contractor relationship is just that - a relationship. Just like most people date around, expect to spend a lot of time interviewing contractors.
Really, until you do a few rehabs you won't really know what questions to even be asking. Your best bet is to work off referrals and talk to as many people as you can. (Protip: The guy who wants money to come take a look may be your future best resource. The guy who comes and talks to you for 4 hours for nothing is probably a schmuck. Really good contractors are really freaking busy and tend to have a pretty high opinion of their time.)
Things to think about...