4Mp anything is a waste of storage space if all you're doing is posting to the web. The 1Mp setting is more than enough. Since most displays won't show more than 72 or 92ppi, an 800x600 will show up as 8.7"x6.5" on most screens. (11x8 on the older screens--72ppi). That is HUGE on the web no matter how you look at it.
For 4x6 prints, 1-2Mp is plenty and 2Mp is good for up to 5x7 easy. At 4Mp-N, a 11x17 is sharp as a tack. If you can't get high quality 8x10's at 4Mp, then something is wrong with your software or printer.