If you have some short songs, just play them through 2 or 3 times to extend the time and keep people on the dance floor.
Actually, you can do this with just about every song. I don't think anyone will be timing you and pointing out the you played it longer the the original version, especially if they are carrying on out on the dance floor.
Also, for a 3 or 4 hour gig, a lot of the people still there at the end of the gig may not have heard your first set, especially the opening numbers.
If you haven't already, you may want to check out the following TB threads for some great band and music advice:
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Glad to hear you're out gigging. IME, just about the most fun you can have with your clothes on (other than driving fast cars). Keep a positive attitude, expect the unexpected and always communicate with your band mates. You'll be digging it for years to come. Oh, wear earplugs. Always.