I just had my mouse give birth and they're all almost 2 weeks old already. The babies all seem healthy and have a nice coat of brown fur. However, mum seperated four of her babies from the rest of the gang today. Four of them she keeps in the original nest while he other seven (which are generally bigger than the rest) are kept outside the nest and she nurses them outside as well. I don't know if this is temporary or not but i'm really worried. What could be the issue here do you think?

Leave well alone. She is coping in her own way. unless she not is feeding them there is no need to worry. Just make sure there is plenty of nesting material for the seperated youngsters.
She obviously is realising that there are some runts in the litter and is seprating them.

Sometimes the mum will abandon the smaller youngsters or kill them if they are not doing well.
Only if she does not feed them, then you can interfere and remove the abandoned from the nest with rubber gloves ,without disturbing her or the other nest and try syringe feeding an orphan formula.

Thanks a bunch. I believe she has 'forgot' them. Because i checked in on her and she has put them back with the rest now.