The late 2015 iMac was released in October of 2015. I don't think Apple is going to release another iMac better then the current flagship until at least the fall.
That said anything is possible. Another updated machine is always over the horizon. Buy what you need now and stop waiting or you're in for a long wait.
Just received my new iMac 17,1 maxed out yesterday. Totally worth it.
I was thinking of waiting for the next release anticipating tb3, usb-c ports among a few other things... but nope. No buyers remorse at all - this thing whales
There's always a new computer just over the horizon with better specs than the current configuration. I pulled the trigger early 2015 and order an iMac (late 2014). I'm 100 percent satisfied with even though I know the iMac (late 2015) has better specs. Get the latest specifications on the current iMac and you'll be completely happy, if you're still hedging and hawing then maybe the iMac isn't what you're looking for?
next update likely october (receiving november). unlikly anything earthshattering. it's a great time to purchase the 5k. i would wait for a revision before purchasing the 4k