The ipad 4 will mostly likely have enough processing power for at least 2 years given it's speed is more than double that of the ipad 3.3 owners can update then. Price of 4 will be lower... and/or newer iPad will be out with new features/speed.
I suppose it all depends on what you use the tablet for.If you are just going to browse the internet or a bunch of photos,then the 3 is good enough for years to come.
The ipad3 is already on the ragged edge playing current state of the art games.It's safe to say once the ipad 5 comes out and new games and apps are released to take advantage of the superior hardware the 5,the ipad3 will struggle but the 4 will keep up.It might be less than a year.
Having said all of this,I probably wouldn't upgrade to a 4 if I had a 3.But if I had to choose between a 3 and 4 right now,I would rather pay a bit more and get the 4.I would then say stuff the 5 and 6 and I will wait for the 7 before upgrading.If I buy the 3 now,it might struggle with a lot of apps in a years time so I might have to upgrade earlier than anticipated to get the 5.If I was going to do that I may as well just wait for the 5 and skip the 3 altogether.