I have a four wire on a 20 Amp service. I need 110 outlet.

4 wire recepticals are used for 220 Volt appliances, there should be 2 hot (117V to ground) and one each Grounding and grounded (read neutral) conductors) Breaker or disconnect should be 2 pole for 220 Volt circuit. A 20 Amp 110/117 Volt circuit would have a single pole breaker or disconnect, one "hot" conductor(should be black or colored , not green or white), one neutral conductor (should be white or neutral with white stripe) and an equipment grounding conductor (should be green or bare copper). It sounds to me like your receptical was wired for 220 V using whatever colored wires were handy. Back up to breaker or fuse (disconnect), and shut off circuit. New 117 Volt outlet will be wired with Black to Brass colored screw (short slot on face), One White wire will be used for Neutral conductor (make sure you know which one it is at the breaker panel) and one of the other white wires may (but shouldent be) used for ground (use green marker to identify it at each end),The third white wire should be removed if you can, otherwise cap with wire nut at each end and coil up in back of boxes. At the breaker/ fuse panel, the Black wire is connected to the breaker (It will be your "hot" lead), The white being used for the Neutral is connected to the neutral buss bar with the other Neutrals, and the green goes to the grounding buss bar (should have other green or bare copper wires. Once all is properly connected and secured but without the disposal plugged in, turn on the power and check for leaking smoke (smoke here is a bad thing) The fuse or breaker should protect your wires and help keep that smoke inside the boxes. Check voltages at the new plug, 117 V accross Short and longer slot, 117 from short to ground (round) and 0 or close to it from long (Neutral) to ground. If voltages check, plug in the new appliance and test.
If in doubt about what you have, dont hesitate to contact an electrician, they get paid for their knowledge and experiance and their ability to keep that nasty smoke inside the boxes.
Good Luck.,