If you are going to eat one of the bulls, eat the younger one. The 4 year old will yield only ground beef as a usable cut--"maybe" the tenderloin as well. Around here, I would be hard pressed to find a processor that would even accept a bull over 14 months old to process. 30 months seems to be the limit--the custom processor has to immacualtely clean everything he uses on older animals due to BSE concers and the head, spine and other Specified Risk Materials have to be removed. If you are in Canada, I think they have finally relaxed the OTM rule but I am not sure. "Over Thirty Months".
The "taste" people talk about form an older bull comes mostly from the amt of adrenalin bulls have in their system. A young bull otoh, IMO, tastes, cooks, and eats as good as any steer the same age and history (genetics and feed program).
BUT, you have only 2 cows--and other than the emotional attachment thing, WHY 2 bulls?
If I were in your shoes, I would get both cows bred, then sell both bulls--buy another cow and going forward, rent, borrow a bull for 60 days each year. You are feeding 2 bulls and getting about 10 minutes use out of them per year.
Will they fight? eventually yes--imo.
The only thing that stopped these 2 from mortal combat was 20' wide 10' deep water and 8000 volts at 6 joules. No females in heat or present at the time. White bull swam the waterway, climbed the bank to get at the red bull and soaking wet, hit the hot wire. He was not happy when he had to swim back across and I took the picture. Good thing I was on a tractor and not afoot.