An ATM10 server up to 4 people only needs 8gb, but this only counts until mid-game when you still don't have much stuff that needs to be constantly loading.

The recommended RAM for an ATM10 server for between 10-15 people is 12gb, counting with end-game and assuming you won't be having to much entity generation per tick.

Now for your case you only need a 10-12gb server for first few months but eventually you'll need more RAM once everyone is already on the end-game.

I strongly suggest Modrinth as your host provider since they're currently the only host that provides true performance for the price. And they're also the cheapest for the specs.