
I would say neutering the older one will only reduce the social distance between them making scrapping for top position more likely.
Accepted wisdom would have you neuter the younger to reduce his status. I don't think I truly believe this as I have known a neutered top dog in a household with several males, all bigger than him.
The four year old has only really just reached social maturity, so being top is a bit new for him and he isn't handling it as well as he might trying to keep the young pretender at bay.
At this stage with just spats it is easy enough to get them to cool it, and I bet they re quite relieved when the true boss stops things. Distract and reduce excitement.
I have found that if there is a disagreement both parties need to be told off equally, don't take sides, a spat takes two.
In any relationships you will get times when individuals will push and test each other, and try their luck.