@Charles Leon here are my 2¢: the idea of doing your flips in a legal entity is related to liability protection. [A trust usually doesn't offer liability protection, just a layer of anonymity.] If something happens during rehab [accident, fire, etc.], you can get sued and you'll be liable. With an LLC (and you can have it taxed as it suits you best that LLC) you have protection (if LLC was properly created and maintained). That is the reason why you want to do your flipping in an LLC.
If something happens after selling the house you held in an LLC (like the seller coming back because the walls crumbled) and you get sued, everything that you hold (like the other 3 flips you got going) in that LLC can be at risk (e.g. a judge can put a freeze on your LLC accounts and all your flips will get in trouble). That is the reason you might want to do each one in their own LLC and not reuse the old used one. Look into Series-LLC if available to you as a tool to mitigate the associated costs to easily create child LLCs when needed.
And if I'm not mistaken, if you close the LLC, the liability reverts to you. So it's better to leave them dormant for a few years, not to close them.
I'm not a lawyer, so take this like any other free advice from the internet. Consult with a specialist - @Scott Smith - to get expert advice.