Who knows, Apple might still release a 24-inch 4K SST Retina TBD now that there are some nice panels available. I would trade my UP2414Q for one if they did.That said, those problems with the UP2414Q are way overblown -- not planning to trade it for a new UP2415Q.
Maybe... a 24" 4K at 180PPI is not quite up to retina display density though. Apple seems to have established 200PPI as the standard for retina computer displays so I see the next iMac refresh including a 21" 4K retina display to go along with the new 27" 5K. When DP1.3 is incorporated into Thunderbolt and offered in MacBooks we could then see a 27" 5K TBD but that could be at least another year away.