Four years old is not old to bring in another kitten. As long as both are neutered/spayed, then you will have less problems. Do the normal slow introductions:
1. Put the new kitten in a room by himself with litter pan, food/water.
2. After 2-3 days, then switch rooms for awhile. Put the resident cat in the kitten room and allow the kitten to roam in the rest of the house (maybe for an hour SUPERVISED). This way they can get used to each other's scent without meeting face-to-face.
3. If all seems calm, you can then put the kitten in a carrier and allow the other one to check them out more. Expect some hissing or growling at first.
4. Before you let them physically together, sprinkle cornstarch baby powder on both and rub it in or use some vanilla extract on their chin and base of tail.
As long as they are not getting too physical and attacking (which probably will not happen) let them alone together in one room while you supervise them. Again, do not interfer with some hissing or swatting.
The older one will let the new kitten know who is boss. Neutered males usually are more friendly and adjust quicker to a new playmate then a female will
One other thing - get another litter pan now. If they share the one, fine, but you don't want problems by making them both use the same one.