J.M.
I co-sleep with my two youngest children, ages 1 and 3.
It's a time where they can be close with their father, who's gone during the day. It eases any stress or argument we've had during the day. It means that if they have a nightmare, I'm right there, and if they need a diaper change or whatnot, I hardly have to wake up.
It's been a wonderful thing for me and my family.
When my husband and I need our alone time, we leave the room and go play elsewhere. It's fun. :)
The way I see it, the child and his father had a routine that brought them comfort and that they both enjoyed...then you came in and demanded that it stop. This is not the way to convince a four-year-old that you are on his side. In fact, placing your needs above his like this may cause problems--behavioural and other--in the long run.
If you must disrupt their routine, do it gently, over time. Be patient, and use rewards rather than punishments. Try to remember that when you have a child (and if you marry a man who has a child, you have a child), that the child's needs do come first...or at the very least, must be understood and handled with care.
Best of luck.