4GB is not a lot of RAM, especially since Windows has to have a piece of it as well.

You know give a VM 1GB of RAM and it will still run somewhat acceptably. You're not doing high performance TSQL development, but rather some basic administration stuff.

Most of the time you only need to run 2 servers at the same time, so it's possible to over allocate your RAM.

I don't really have an idea of the RAM usage of Windows Core, but I can imagine it uses less because it is a stripped down version of the OS.

Not sure you can RDP into it. You basically have a cmd module and a powershell module. To manage SQL Server however you can just connect with SSMS from another computer, so there's no difference there.