Is it possible to convert a two light circuit into a four light circuit (one switch)?
The circuit you are talking about is probably rated at 15-Amps.  The electric math looks like this:  15 (amps) x 120 (volts) = 1800-watts.  It is always a good idea to derate your circuit at by 80%.  (This gives you a margin of safety.)  1800 x 80% = 1440-watts. All things being equal you can install this kind of wattage on this circuit.  It doesn't really matter how you do it.