First, I personally would change every 'him' to 'he is'. Sentences 1-4 are all fine except for a spelling error in 4.
Q) Is this possible?: It is believed that Bill is taller than him.-- Yes. You do realize, I hope, that John disappears from all of your sentences except 1 and 2. 'Him' becomes anonymous.
P.S. In the question, which is better idea to put a quest