They're much better. Depending on the streaming service, 4K video is transmitted at data rates between 20Mbps and 40Mbps. 4K Blu-Ray delivers much more: up to 128Mbps. And it delivers that data reliably: there's no network congestion to slow down your stream or make it buffer just as you get to the best bits.
That's significant, because the lower bandwidth of streaming means compromises have to be made. For example, Dolby Atmos on streaming video usually comes in the compressed Dolby Digital+ format. On Blu-Ray its full quality Dolby TrueHD. If you want DTS:X, which has more control and detail than Atmos, that has to be on a disc.
The big exception here is Sony's BRAVIA core, which is exclusive to its BRAVIA TVs and offers Blu-Ray equivalent data speeds. There's also the highly expensive Kaleidescape streaming system, which offers incredible quality, but is not priced for the mainstream.
The selling point of streaming is convenience and low cost (although some services charge more for higher quality streaming), and it definitely delivers instant gratification. But when it comes to the ultimate AV experience, Blu-Ray can't be beaten.