I have installed 4.12.1 over my ModAct 5.12.
It does work.
We set up Battle-Fields 2 server for 4.12.1 and I can enter the server.
Battle-Fields 1 server is still at 4.12 and I get the different version message.
This server uses 4.12.
So with that no matter what the splash screen may say it is 4.12.1.
I hope this helps.