If you're looking for better sound quality, I'd suggest saving your money for decent external mics instead of purchasing an entirely new recorder.
Sony PCM D50 ($484): Good built-in mics. No XLR input. Great for "on the run" recording but the lack of XLR input (and ridiculous size/cost of the XLR adapter) makes it a poor standalone recorder. IMO, not a great choice if you want to focus on quiet sounds (i.e. foley) or plan to eventually use external mics.
Tascam DR-100 ($313): Decent built-in mics, includes XLR input, arguably better preamp/controls/battery life than the H4N. Good as a standalone recorder and also good for capturing quick sounds on the fly. The downside: As with the H4N, you'll likely want a mixer/preamp when using external mics (read here and here). This will run you another $350 or more.
Marantz PMD 661 ($599): Larger size, poor built-in mics, but in other ways a much better standalone recorder than those listed above (preamps, controls, XLR input, etc).
Tascam HD-P2 ($699): Fantastic as a standalone recorder, but you will probably never use it without an external mic hooked up... making it useless for quick and easy "on the run" recording.
It's a matter of weighing your options. Do you plan to eventually use external mics? Or do you just want something small to run around town and record sounds with? What kind of sounds are you looking to record?