Dell's 24" UP2414Q is an IPS 4K monitor with 99% AdobeRGB and 100% sRGB coverage, and it has a 185 PPI panel that I'd argue is already in the "retina" range. Although with its ~€555 price it's not exactly in the $300-$500 range, but it's a close call, so I wouldn't call all 4K monitors around this price range "shitty" at all.
Update: it works in 60Hz @ 4K.