After about an hour cortisol levels rise to a point where you really just being counterproductive. Cortisol is a muscle-wasting hormone. Training should be intense, and purposeful. Don't be chatty with people, take too much time between sets, etc. If my training partner doesn't go and I work out alone, I'm usually done in 45 minutes.
Your body doesn't grow in the gym. You grow when you rest and recover.
Also, a quote comes to mind. I believe it was Robbie Robinson who said, "You want to stimulate the muscle, not destroy it."
What are you doing in the gym that takes four hours? What does your training split look like? How much cardio (if any)?