At four months you are safe to start having sex again. Though if you are asking because you dont want to get her pregnant again, then she can still get pregnant without having her period back.
I had a baby 3 and a half months ago and my period started again 5 days ago.
We were told that breastfeeding delays your period and can be used as a form of contraception though its not 100%.
You should still use some form of protection if you dont want to get pregnant again, though you dont have to wait until she gets her period to start having sex again. Unless the doctor has said she should not have sex for some reason then your good to go.
That question your wife should ask her gynecologist. As only he knows if it is completely okay in her unique case. Such a procedure as postnatal examination should have taken place within the 2 months after delivery and the doctor should have told your wife if she is ready to resume sexual activity.
Normally 4 months after delivery is a sufficient period to resume sexual activity. But you should be careful and avoid rough sex at first. Be delicate.
Regards, John.