But was it fresh? If you mixed 8 oz (or whatever quantity), then fixed 4 rolls of film consecutively, the fixer might have been close to exhaustion.(posted 8480 days ago)Or, did you fix all 4 rolls simultaneously? If so, were all 4 rolls covered with solution? (You actually have to look in the tank; the measurements indicated on some tanks won't actually be enough!)
If you describe your process and how each of the 4 rolls was handled, you'll either figure it out yourself as you try to describe it, or give us enough info to figure it out.