Is there a "San Juan" in Costa Rica? Or did you mean either San Juan or Costa Rica? Also you never mentioned when you want this vacation to take place. That is critical for award availability.
110,000 miles is a small fortune for one, and is grossly inadequate for four people. (That is only 27,500 miles per person) Do you have miles or points from other programs that you can transfer to the United program? For example, the Chase Ultimate Reward points can be transferred into the United program, and you can use it to pump up your balance.