Can't you kids just play nice?

My two youngest children cannot play together for longer that 15 minutes without fighting. I try to turn the t.v. on so they have more things to do in their room than just play. My 5 year old picks on my 4 year old constantly. I don't know what to do anymore. I try and seperate them but that still doesn't work. One of them will walk by the others room and see them playing with one of their toys. I just don't know what to do....help, I NEED SOME ADVICE.