Perhaps along with Caryl, I think it's best to keep it simple.
Just keep the tank running for a few more weeks, do the odd water change, and if there's no great spike in ammonia or nitrite then add your next 4 fish.
I don't use carbon, that's more about not wanting to buy more every month! If you think it helps your tank look clear, and you don't mind replacing it, then there's no harm keeping it in.
Don't over-complicate things. Slow stocking, with hardy fish like platys, and regular water changes should be a successful way to start a nice community tank.