An approximately $5-6 4K iMac? That doesn't make much sense. Attaching as sky high priced 4K display to an iMac is close to almost tripling the price with the display.
If there is no new iMac LCD panel to leverage it doesn't seem likely there will a new Thunderbolt display/docking station off of a panel that relatively extremely low volume. There are several 3rd parties that can fill the relatively low volume deamnd for 4K displays with devices that simply just take DisplayPort v1.2 input. Plug into Mac Pro and done.
What the Mac Pro is seriously missing is a 21.5" display/docking station, not a 4K one. The minimal Apple display being priced at $999 is high.
You leaving out the 3rd issue, that is even more significant. At present the 3840x2160 (4K) displays are 31.5" big. A 4K display would require a new physical case for the imac. It is unlikely Apple is going to build a 3rd physical sized iMac. ( there was a brief time during 2006 when Apple had a 3 sized screen line up, but that was a huge outlier. There was no were near the gap between the two top ends either. )
There are people in business over the last 3-4 years that required 10-bit displays and studio reference monitors. Apple didn't make those either. There is a huge difference between Apple making something and folks in business being able to buy and utilize it. Right now 4K is relatively small and the monitor ( not docking station ) vendors covering it are for the most part covering the market.
When 4K prices dramatically drop in 1-2 years then there is a play for Apple because demand will go up significantly and there is room for more players.
Until 4K shrinks to 27" and/or 21.5" sizes and current 27"/21.5" price points it probably will not be going into iMac. Doubtful that happens next year.