[crazywiggy]

I personally recommend getting him a friend. Rabbits are a social species, and really need the company of their own kind. It is pretty rare to find a rabbit that really prefers to be alone.
I would say that human company doesn't actually make up the company of another rabbit - but even so, as you work full time a second bunny is even more important because otherwise he is completely alone.

4 isn't particularly elderly for a rabbit, so neutering is still a fairly safe option, and I personally would recommend it.

As for a second rabbits, I would suggest getting him neutered first, then adopting a spayed female. Male and female does tend to be the best pairing for rabbits. Male rabbits are usually obsessive about humping (even toys, people's legs, my dog!) so I doubt it would matter much if the female was spayed, he would almost certainly try to mount her anyway unless neutered himself.