Just 4? I have the same problem as ireckon in being something of a knotwork junkie.

1) Tautline hitch - Good for tensioning stuff out when you're not expecting weather.
2) Truckers hitch - Good for lashing cargo and tensioning stuff out when you are expecting weather.
3) Water bowline - Varient on a bowline where the full turn above the bight is replaced with a clove hitch. Add a stopper knot (double overhand, figure 8, or Ashley) to the bitter for additional security.
4) Single/Double/Triple Fisherman's knot - easy to tie, but not as strong as a blood knot.