You should wait at least until her stitches are out and she is back to normal after the surgery.
4 Months is quite young too, I had to wait until she as at least 5 months with Molly to make sure she was developed and the surgery wouldnt be 'pointless' so to speak. The vet suggested 6 months was best but I wasnt sure exactly how old she was at teh time either!! Being a rescue bun they thought she was about 3 months when I brought her home.
I actually bonded Molly with Charlie before her op so was very anxious to get it done as soon as poss so she showed no hormonal behavious - wrong way round to do it!!! All was fine with my pair - I was lucky that she is so submissive with Charlie, adn that he wasnt too OTT when bonding.