If you don't want to spend the entire time on the beach I'd go for Guatemala. Belize is very nice, and has other attractions, but beach resorts are their niche. If you go to Guatemala, you can visit Guatemala City, Antigua, Panajachel, Xela (aka Quetzaltenango) Flores / Tikal, Semuc Champey, and if you want some beach time Livingston. Check out Playa Blanca. It's very nice. There are a lot of nice cultural experiences in Guatemala to enjoy.
Go to belize. check out a place like www.cavesbranch.com they can take you to the beach for a little while but culture and history mixed with adventure is their specialty. Belize's official language is English so you will have no problem talking with locals and getting around in the country. spanish is widely spoken as well but most people are bilingual.
Both countries are interesting.
But if you have only four days to spend there, you'll be better off choosing one instead of trying to see both in such a short time.
If you speak only English (not Spanish), you'll find a higher percentage of English-speakers in Belize. It's good to learn Spanish, too, but four days is not much time to learn a language. On the other hand, if you aren't going until March, you have almost six months to study Spanish before then.
I am planning a very quick (Friday night through Tuesday night) trip for the beginning of March. Flights to this part of the world seem relatively cheap ($440) and I want to know if I should go to Guatemala, Belize, or both!
I don't want to spend the entire time on the beach - I want to see culture and have an amazing experience! Which place should I go, what should I do, or should I maybe do both?