Well that's the beauty of mutts -- they're a surprise! It will be hard to tell from guesstimating alone, but knowing both parents I would assume to just take their average weights and maybe factor in if they have more of the mom or dad's genetics... You can take 100 pit/lab mixes and I'm sure they'll be all sorts of different shapes and sizes

4 months and already 30 pounds, though, my 3 year old Aussie is 40 lbs! So to me she'll probably be big, hehe.