[imacomputertoo]

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,

[ljm]

You might have some success if you run the build before deploying it (provided you use the same arch as the droplet). It's not going to give you a lot to work with and Puma's default config will take up most of it.

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.