Yes, memory. 4$ droplets have only 512mb. that's barely enough to run bundle install for one application.
6$ droplets give you 1GB and that's plenty for one app,
6$ droplets give you 1GB and that's plenty for one app,
A 1/2GB box could run the bare minimum if you host the database on it as well. For a small scale app or a hobby/toy project that's going to do just fine.