Hire a professional trainer if you are unable to deal with 2 male dogs in the home. Most people get opposite sexes when they get pets, so as to not have to deal with this male dog aggression. Train them.
...about 6 months ago.? The 4 year old is unwilling to go out with the 6 year old. The 6 yr old's aggression has increased and he has also growled at my husband when he tries to stop it. Any suggestions?
The 6 year old needs drastic help if his aggression has escalated to growling at people. I'd suggest getting a behaviourist to help with this dog because there's more going on here. A 6 year old dog doesn't suddenly show aggression to his owner, he's had a problem before which wasn't dealt with.
Professional help would be the way to go here. You need to actually be able to observe the dog and watch the interaction going on in the household.