| You can buy 4 string sets and be fine. Most people buy strings according to preference of feel, so any suggestions here are should be taken with a grain of salt. I personally use an .105 E string but have been meaning to move up to a .110 4 string set for a while now. If you like the sound you get from the strings you use and they don't feel too floppy, then you should probably stick with the same gauge.
Other wiser TBers should correct me if I'm wrong though.
Is your 120 Watt combo amp going to be your only amplification? Most venues will have PA systems for you to run through, so your personal amp wattage is often moot. The answer to your amp question also depends on how many other instruments you'll be competing against. I had a 120 watt 15" combo that I used for a lot of shows, but I wouldn't have trusted it to work adequately in a 500 person venue on its own because I had to compete with 2 guitars running through 100 watt tube amps. | 
