New Lights, but need a little help.

OK, i am looking to buy either the D Lite 2 or 4 kit
This is for indoor portraits where i would set up shoot and dismantle when finished.
I do it for fun/enjoyment, people seem to like the photos i have hanging on the wall at home and have asked if i could take shots of there kids etc.

So how do i decide which lights would be most suitable for this.
Obviously one set of lights is more powerful than the other, and they are both now fan cooled, so i need to know what questions to ask myself so as to get the right lights first time around.

My thoughts are for getting the 4s, as you can always turn them down, and move them further away from the subject (right)?, where-by the 2s if your on full power than that's it your stuffed.

also what would i use for a backgroud light/hair light ? any suggestions, most kits i see usually have just the 2 heads, so you need to buy a third head seperatly, is there a reason for this?.

thanks for looking.

Generally you want your lights as close as possible to the subject for soft light, so moving them away is counter productive. If you have more power just close up your aperture. It's rare to have "too much" power, so get the 4's. AFAIK hair lights don't need a huge amount of power so you might get away with a small slave flash. Get the kit and experiment, that will tell you what you need for a hair light.