Doug,
1. Square knot: simple, strong, easily tied.
2. Two half hitches: a useful slip knot, easily tied.
3. Bowline: a good stable loop, not that hard to tie.
4. Taut line: very useful to tighten tent & other guy ropes.
5. Sheetbend: a wonderful splicing knot. I know you only asked for 4, but this is an old favorite to which I frequently resort.
Good luck,
John