You could probably run 4 600w lights with that generator safely. You would want to sragger the time each light comes on by a few minutes because they really draw a big surge when they first kick off. A generator that size usually burns about 1/2 gallon an hour maybe 1 gallon on heavy loads. So your talking 4 to 8 gallons per 12/12 cycle so about $12 to $24 a day for gas.
Your one 600w should have only increased your bill by about $25 a month.