Yes, they can technically rend twice. Girallons are the nastiest CR 5 creature in the book. A half-fiend, flying (with half fiend bat wings) that charges up Poison before attacking is an especially nasty encounter. With one more hit die, they gain an extra feat. May I suggest Flyby attack? Since all 4 claws are listed together, a girallon will get all 4 claws even on a charge action. Flyby attack girallon is very memorable, trust me.
I stand corrected. My rules lawyer friend told me wrong. Thank you Dr. Zoom. Of course, you could haste the girallon...