The real question is whether to wait for the iPad 5 or wait even longer for the iPad 6...
You can spend your life waiting, or you can just get on with it. I recommend the latter.
While that's generally true, the next iPad is likely going to be a gigantic leap compared to the prevus updates
IPad -> iPad 2 - Big leap. New shell, drastic NOTICEABLE speed improvement
IPad 2 -> iPad 3 - Small leap. Retina screen is amazing but thicker and no speed improvement
IPad 3 -> iPad 4 - Small leap. No new features. No A6X apps. Same old design
IPad 4 -> iPad 5 - Huge leap. (speculated). new design to match Apples current product line (bonded glass, thinner bezel, lighter shell). Improvements to screen. Stereo speakers. Additional hardware (NFC? GPS in wifi model?). An OS that actually deserves to be on Apples amazing hardware.
Peopl are strongly defending the iPad 4 now, but my bet is that when the iPad 5 comes out it will be obvious that both the iPad 3 and 4 are interim "filler" products that were simply biding time for Apple to "get it right". Apple is overdue for a revolutionary upgrade so I'm feeling that its going to be in Q2 or q3 rather than Q4 again.