I'm running Pleroma and it's built in Elixir, which is considered a spiritual sucessor of Ruby on Rails (There's more in-depth explanations on the web but I considered this article to be very good due to its historical and technical comparisons https://curiosum.com/blog/elixir-vs-ruby-comparison).

4GB seems fine for Pleroma. My VPS has 1 core and 1GB RAM, running ubuntu server. It looks like Pleroma uses ~700MB which is pretty good but I didn't use Dockers and installed it manually, which sped things up. Mileage may vary as memory can RAMp up (pardon the pun) the more follows and followers you have.

A word of caution using Pleroma, as there's preconceptions attached to it. As far as I could gather during my search on the topic, most of its developers are “free speech absolutists” , which tend to cater to unsavory people, so there are admins who consider all Pleroma instances to be bad, regardless of its users, and tend to block them. I don't want to bore you, there's extra info on my introduction. I found this mostly from word of mouth and reactions from people so I can't be of more help explaining the full problem.