i can tell you're on the right track as to what you're thinking. like i said though, 4000k isnt putting out just one color. its putting out different colors of different values and the color temperature i believe is an average value of what we, the humans, can actually see. So, you're bulb would appear white and not blue like a 6500K lamp. That we know. But does it put out the significant amount of blues that a 6500k would do?

http://www.plantlightinghydroponics.com/eye-400w-6500k-full-spectrum-horizontal-metal-halide-bulb-p-93.html 400w 6500k MH

HydroBuds. 400w 4000k MH

and keep in mind they aren't made by the same manufacturer, so different product different results. and to your question about the cfls... i still think the wattage/"lumen" ratio for the MH would be better but still completely unnecessary since you already have plenty of light for a few small plants. thats other than the sq. space factor is all i can offer.