Possibly the most interesting of the new players in the Philips BDP7501. It is tiny, moderately priced (US $400) and supports the new HDR standard. HDR is short for High Dynamic Range, meaning the pictures have more and clearer colours, deeper blacks and are brighter and more lifelike. Although manufacturers and pundits like to emphasise high resolution as the key to better video, in reality resolution is only one aspect of how we perceive picture quality and not actually the most important - HDR is a lot more significant a development. Note that to support HDR, a TV set must support HDMI 2.0a and many of the low cost 4K sets currently on sale do not do so. The new 4K Blu-ray disc standard incorporates HDR (although not 3-D - anyone remember that? Not to worry, it will make another appearance in 20 years time).
The BDP7501 also supports both VP9 and HEVC codecs. This won't mean much to the person trying to sell you a 4K device in the High Street stores, but it is important. These codecs - essentially software for encoding video - are used by Netflix, Amazon and YouTube to deliver 4K video. Many of the current generation 4K UHD Smart TVs do not support them, meaning that even though they may have, say, a built-in Netflix app, it will only handle regular HD rather than 4K.
It states above that the BDP7501 is 'moderately priced' at US $400 (maybe £299 in the UK?). This is, of course somewhat more expensive than a standard Blu-ray player, of which perfectly good examples can be picked up for around £50. However, as with Blu-ray and original DVD, prices can be expected to fall rapidly. Typically, new technology is available from the big name companies for the first year or two, after which the 'no name' Chinese manufacturers are allowed to licence it. Next year, expect 4K Blu-ray players at less than £99 and in a couple of years all new players will be 4K and with little or no price premium over today's standard Blu-ray ones.