My 4 yr. old male cat has been attacking my 8 yr. old male cat. There is a female 4 yr. old (unrelated) who stays out of it. Why is this happening and what can I do about it?

two males and one female ...
I hope your males are fixed :) even if the female is spayed, two not fixed males would fight for the girl :)

If the boys are fixed - I assume that's the case, you basically just could try to socialize them by lot of playtime, rewarding good behavior with treats, and distraction - throw toy(s) along side the cats to distract them) and reward them if they stop fighting and start playing with the toy(s) instead.
you could also try offering them activity toys such as the slimcat ball > http://www.amazon.com/PetSafe-SlimCat-Fo…
this will keep them active and entertained (they have to work for their treats / kibbles) which could help reducing their desire to fight

Also a cat pheromone diffuser (e.g. feliway) could help calm down your cats.

Reasons why they fight could be the following (just to list up few ...)
- lack of space due to small home (provide cat a medium sized/large cat tree and/or a medium sized/large scratch post to give them more place and surfaces to sit.)
- lack of toys (more toys - ping pong balls, toy mice, catnip toys ...)
- boring days (activity toys will keep them active and will reward them by earned treats provided by the toy)

Also in a multi cat home I like to suggest to get a good book about cat behavior and how to correct unwanted behavior - just to have help handy if needed for whatever reason.
a list with good books >> http://www.amazon.com/s/ref=nb_sb_noss?u…