I would say definitely wait for the new one to come out. The difference this time around is not just the processor bump and maybe ram, but it will also have a new design as well as being lighter. The new design is basically the same exact design as the iPad mini, but larger scale. I personally would wait for the new design as I think it might be more friendly for use, and the shaving of a few grams definitely helps you enjoy the experience. Granted, even though I think it would only make maybe 20% of a difference if you got her the new 5 instead of just getting her the 4 now, I still think it is worth it for your mom's birthday gift. That's just me personally.
I do not think the iPad 4 (iPad with Retina Display) will be that much cheaper after release. Might go down $50-100 (they do hold their value), but that is assuming they discontinue the iPad 4 (and you buy left over stock on amazon).
On a side note, they may release the iPad's September 10th! But they may also only release them mid October, so it would be a pleasant surprise if you got her a device that came out literally like 3 days before, but it might be cutting it close. They would announce the date they're selling them at the October event (sometimes they sell a week later, sometimes right away). Good luck