Most Helpful Girl
The Only thing you tell your innocent little four year old is, "I'm sorry, honey, but daddy/Joe is not with us anymore. He had to go away, and from now on, isn't able to be with us." As he gets older, with any luck, he will have completely forgot about him, or perhaps later learned all about the kind of "male figure" that was in both your lives. It's not necessary to do it now.
Next, go to court and get a restraining order against this loser. This will not only keep him away from You and your son, but at the same time-----keep this filthy douche bag from continuing to dump his own "naughty" on You, and to stay the hell in his own parking lot, whether it is at home or in------his family business.
This schmuck is a two-timer and an all time waste of your time and precious space, sweetie. He's never going to change, and in fact, I bet my bottom buck with this type, will only get worse. I don't know How you could have found the "no bitchy" nice spot in your harnessed, harassed heart to even take him back all those times. Perhaps for your boy's sake.
You Do Need to "move forward," in fact run like hell while you're at it. And don't ever look back. At this point in time, I would rather be dirt poor and homeless, living in a cardboard box, than to be Ever again in this Lifetime, in the presence of some "disrespectful piece of slime" such as That one.
Good luck, and start mapping out your life tomorrow. xx0