In any case, do make sure you mix the beejeebus out of EVERYTHING before you attempt this. It would make me hella nervous, but you don't want one good quart and three bad ones. That will make you crazy.
i also recently bought my first gallon, and i had this same idea. the problem is, i am pretty sure once you add water to diazo, it gets activated and starts its life cycle (i'll let the pros prove me wrong, if not). the only "true" way to make this happen would be to pour the powder onto a scale, divide that by 4, and only add water to the portion you're going to mix.
so what i did is just mix the whole damn gallon, and i put the other three quarts in the refrigerator. i'm on my second quart now, and it's working out fine so far. at this point, i don't care about the other two because buying a quart at the store costs $31 and buying a gallon costs about $56. so the way i see it, if i can get through 2 quarts, then it doesn't matter if the other two go bad. i'm still saving money
you are right of course, but like i said, if a quart costs $31 and a gallon costs $56, then you should always buy the gallon if you know you're going to get through at least 2 quarts of it. sure it's wasteful if you don't use the whole thing, but the other option is to pay $62 for two individual quarts, which is flat out stupid if you're at all concerned about money.
i'm going to see if i can get through the whole gallon, but if i can't, i might consider donating future portions to my local CC (if they'll take it).