You would want a minimum of 4 lamps, 6 or 8 would be lots better. 4 bulbs is not much light by reef standards. I run that on my 90 gal fw planted tank.
Carefully consider what your doing here. It might be far less expensive to just buy new fixtures, since they usually come with bulbs. Purchasing the replacement bulbs. that you are almost sure to need, will likely cost almost as much as just getting a new fixture.
Also, what do you have now? If it's a two bulb fixture to start with, your talking some major upgrades to used it on a reef.
