I would strongly advise against placing all 4 brothers together - males 'teenage' months when they have strong hormonal surges and therefore are more likely to fight with other males is generally from 3-15 months of age. To have 4 pigs going through this stage together is almost guaranteed to end up with fighting, injuries and potentially 4 singles males that will not tolerate living with another male.
The pigs would be better off being kept in pairs, though you may be very lucky and find that if they are housed with dad that they will accept him as the dominant. Very few trios make it through the hormonal months so be prepared to split them up again in the near future.
Please do not house the males too closely to mum, her scent especially during her season (every 17/18 days) is likely to trigger dominance issues with the boys and the possibility of a fall out is higher.
Mum will need lots of feeding up to recover from back to back pregnancies and, hopefully, a female friend of her own.
Suzy