From what everyone has said, HPS is worth the extra cash for the improved yield. I mean, if you get even an ounce more because of the HPS, it'll more than pay for itself, even if you sell that extra ounce to your friends at a deep discount from street prices. Your flowering light is the one thing you definitely don't want to skimp on, and you really should use HPS.

What you have is probably fine until you're ready to flower though. If you can, try and use a bluish tube in the 4' fixture, like GE Daylight, which has a 6500K spectrum. That'll keep the plants from stretching too much. Most but not all CFLs have a very yellowish light with a spectrum of around 3000K. Do you know what yours is?