4-8" ? no you shouldn't need to transplant them again. Are they root bound? Do they need to be soaked every 2-3 days, if so then maybe, but usually at 4-8 " a 16oz cup is just fine.

And sure - if you have more plants than you are going to use, and no one to give them to then you might as well cull some of them now, just to save the work.

How do you know when? Is there a rule of thumb?

Agree with mandolls. But no there is no "rule of thumb". Just as with watering it is when the plant needs it not by the size of the plant.

And you know when by the fact that you have to water them more and more often to keep them from drying out and drooping. That is the sign that the plant is getting rootbound in its current container.

Dave

what i did, and i have a large area for indoor growing, is cut off the top, and replant into 1 gallon grow bags, with the entire stem buried up to the top 2 leaves. now the entire stem will sprout roots, and you get a really husky "seedling" - you just really need adequate indoor lighting to do this - :) - paul m.