Actually Broadwell promises lower TDP and Better integrated graphis and about the same performance as Haswell. so no case being Haswell or Broadwell in regard to 4K display.
Right now Haswellś iris pro 5200 have plenty power to drive 4K displays, aswell nvidia 980m being superior than 880m which currently support 4K output at 60hz its logical will be much better option.
If an iMac refresh is released this year will have Haswell or Haswell-E to drive a 4K or 5K screen (27-28'model), no chance next 12month t see an Broadwell desktop unless its based on mobile cpu.