it's doable in an 8ft tank but no guarantee
There is plenty of information about the average size of a mated pair of clowns territory online...
You have good sized tank for it, make sure to keep this in your mind while you are aquascaping.
Having ideal areas for a clown to host something that is out of the sight line of your other pair should go a long way in minimizing aggression.
That being said, it's a risk and one that has not worked out for many hobbyists.
http://www.reefcentral.com/forums/sh....php?t=2002709