For only $79 from the App Store there is little reason to wait. I would expect A4 to be another year out (unless they really are planning an update next month and that is why they dropped the price to less than half). Adobe product like Photoshop and LR are predictable upgrades about every 18 months while Apple is on its own secretive schedule.
Another argument for waiting is that A3 was released too early and had many bugs like memory leaks. It appeared to be more of a beta than a final release - which was very surprising for Apple but that story is not to far off from iPhoto 11 which makes it seem like Apple is not putting everything into some products unlike their iToys (ipad, pod, phone...).
I did put my money where my mouth is and bought A3 last week. If A4 came out I would not jump on it until the early reports of it working perfectly or being buggy are out. I also suspect that if they keep this new price of $79 that future version will cost the same instead of a less expensive upgrade to a current version - this is what they do with iWorks and iLife. No upgrade price, just full price but a reasonable full price.
Also, lots of people complain A3 is still slow and opens slowly. On my 2010 MBP with 8GB RAM it takes about 5 seconds to open the first time after booting up and so far performance has been fine but I haven't set up large libraries yet. So it might help to have plenty of RAM to avoid possible performance issues.
