I believe they said III would be the last one anyway.
Originally Bespoken by
Necronopticous 
Originally Bespoken by
Ouch! XS4 just wouldn't be the same without Spoiler: Jin, KOS-MOS, and chaos anyway.
And, more importantly, Albedo.
Actually,
Spoiler: While Jin is definately Done for, Chaos will definately make another appearance. I would suspect to See Kos-Mos is some form or another as well, given her relevance and history. And I wouldn't be surprised if Albedo would rise up in Jr. from time to time, in some form or another. He and Jr. did unite in the end after all.
This is all if a fourth was coming, which it probably won't.