Just my thought, but,
I thought that with MH, the wattage directly relates to the intensity and the depth the light will penetrate in your tank. I would not think a 50 watt bulb, even if you had multiple bulbs, would penetrate the water very far.
If you already have 4-55watt pc in a 40 gallon, I would think that is the maximum light that is going to give you anybenefit, without upgrading to MH.
I would either stick with what you got, or upgrade to 250watt bulb MH.