my sister an i have different dads she is 13 years younger than me. we told her when she was about 4, too young to really understand but just knew the facts, then as she got older we would talk about it from time to time and she would understand more each time.i think this really worked for us as it was never a surprise. it was always just known and wasnt a big deal ever. if you think he could grasp the basics then maybe now could be a good time, maybe you can avoid the whole shock of the facts. at the end of the day, you know your boy and if it feels right then go for it.
i doubt that at 4 your ds would say that wants to see ex, maybe something to deal with down the line...just speculating though.
hugs.