Hi Carol,
Great question. And sounds like you’ve done a lot of good research already. First, it’s a good idea to get a quad band phone to be sure it can be used in Costa Rica.
As far as 3G or 4G, the 3G network is found throughout the country. 4G is being installed now but is not widespread yet. From my understanding, a 4G phone will switch to 3G if 4G is not available in a certain area. So I think you’re good to go with either type but you might get a 4G so you can take advantage of the higher speed network when it’s available.