As far as I know, if you have a dual boot setup, when you choose what OS to go into, it will use the 8GB available. You dont have to split it 4/4. You're only using one OS at a time, so whichever you choose on booting up will use the 8GB you have. Yes it is "undoable" as such. There won't be a button to undo it just like that, but there will be plenty guides out there I'm sure.