First of all, Welcome!! And congrats on deciding on a destination wedding!! I don't have much information about the specific resorts you are looking at, however, for us choosing the resort was the hardest part of everything so far. What we did was make a list of what was and was not important for us. For example, resort size, location vs airport, restraunt options, proximety of excursions ect. From there we narrowed it down to about 3 resorts. We then went to a travel agent who was able to provide us with approximate prices for each resort, and that ultimately helped us make our decision. We were concious of the price as we knew our guests would be paying to come, but at the same time we wanted to go with a resort we loved even if it was a few hundred dollars more.
As to the planning your wedding in 4 months, I beleive it can be done. However, I know the dates you are looking at are really busy for destination weddings. What you could do is contact three or four resorts you are considering and find out if those dates are even available, that might help guide you to a specific resort as well! Good luck with your planning!