It's really hit or miss. When it's something that is depended on knowing when the last ich fell off and when the last cyst actually released. 4 weeks is cutting it close, but there is an outside chance that there are cysts that haven't released yet. I don't believe that anyone can tell you one way or another. If it were me, I would restart. But, that's a long time. Weigh your options on both sides, "Am I willing to fish the fish out again if it didn't take?" better yet, if you have a set-up QT still (you mentioned it busted) leave them in the QT for another 2 weeks or so for observation.