55 watt florsent is about 48-60 inches long
now the cpf are 55 watt replacemen incan, or 13 watts flor. so you should need about 3-6 per sq ft.
I would go with the full service , full service 40 watt bulbs....NOTE full service mag ballast not wattsavors.
Look at my gallery for some 4 ft plants from full service ballast and full service bulbs.
It will be much cheaper over a year grow time. PLUS you will need to replace the cps every 3 months, to maximize the output of the bulbs.