Certainly wait until you find that 1 hour 4K movie takes more than 1 hour to play or you find other things about the 4K experience unsatisfactory. Nobody can tell when YOU will find that that's the case. Depending on what you find unsatisfactory and your research into the why, you can decide what to update.
I have a 2013 Custom Build. It has a Gigabyte Z87X-D3H-CF motherboard with an Intel i5-4440 3.1 (quad) CPU, 6.0GB/s SSD and a 1TB backup hd (unsure of speed), no soundcard or DAC, NVIDIA GeForce GTX 780 GPU, VP2770 1440P calibrated monitor, 16GB of DDR3 RAM, a DVD writer and a brand new Blu-ray reader/writer that will be capable of playing the 4k blu-rays when I get the monitor for it. How long will it be before upgrading for 4k reasons is reasonable given the fact that everything else is running like lightning and 4k content seems to be as rare as diamonds.