I think the 4 month old buck is too big for the little ones yet. I wouldn't put them together. I guess you have been bottle feeding them...not nursing from the doe..right ?? I have just put 2 does and their kids into one big "nursery" pen yesterday...but they are all the same age kids by a day or 2 and their Momma's are there too. Keep them separte for a while yet...until the young ones can get away and butt back a little. I don't think they will mate at this early age yet. Can you partitan off a part of the bucks pen to separte them for now ?? Good Luck !!