So far this has been harder than I thought it would be, the good thing is that it is forcing me to learn at least some of the notes on the fretboard which cannot be a bad thing.
I've also been going through the riffs I know to see how many notes they use, and I think Billy Gibbons from ZZ Top is the 4-note riff wizard. The main riffs to "Just got paid" (A D E G) as well as "La Grange" (A C E G) are 4-note wonders. You can even get a good version of "Sharp Dressed Man" (Bb C F G) going if you leave out the notes played on the G-String.
This gives me hope that I might be able to come up with something that is at least a little bit interesting.