How many CFL's are you using? You have to remember, these plants love light...and need plenty of it to grow to their maximum potential. If you're using several CFL's (3-4), you can expect a slow, but gradual growth. If you can afford it, and have a way to at least supply some ventilation, then I'd recommend going with a 400 or 600W HPS. The color spectrum in HPS bulbs is suitable for both vegetative and flowering stages, and will produce a dramatic improvement in growth.

As for your question, if you've managed to grow an eight inch plant in under four weeks, then you're doing fine. Of course, HID's would make them grow faster, quicker...but that's just not an option for everyone.

Good luck with your grow!:rastasmoke: