TF-4 must be dissolved all at once. When you do, the solids in the concentrate go away and will not re-appear. If you try to mix just part of it, you might not have the right amount of the solids in the mix, even if you shook the concentrate bottle well before measuring off some of it.
I buy the bottle that makes one gallon of working fixer and mix it all at once. The fixer can be reused and will do about 50 rolls of film. The instructions claim much more, but in my experience manufacturers usually greatly exaggerate fixer capacities if you want truly archival results.
I have created a fixer log sheet I tape to the wall above my developing sink. I check off each roll of film I do, till all 50 have been checked off. Then I make new fixer and print a new log sheet. Its free for anyone to use, download from my website in PDF format:
Fixer Log Sheet