The distros that everybody builds off of are Debian, Fedora,Arch, and maybe SUSE ( common roots with Fedora but long ago ).
I did not mention Ubuntu as Ubuntu is actually built from Debian but actually Ubuntu is the most popular and is itself used as a base by other distros ( most notably Mint ).
If you are looking for an Ubuntu alternative, Debian is the most similar. However, pure Debian is not as new user friendly.
Arch is considered an advanced distro. I think Fedora and its derivatives are solid choices.
If you are really running on a system with only 4 GB of RAM, I would actually recommend trying out a 32 bit distro. The 32 bit version of AntiX or the 32 bit version of Q4OS with the Trinity desktop are the two I would recommend.
I was recently reminded of Adelie Linux though and have been meaning to try it on an old system myself: www.adelielinux.org/about/