4K doesn't really say anything about image quality, just the resolution of the picture, which tells you the theoretical maximum level of visual detail.
Focusing on resolution is like asking "how strong is one meter of rope" without talking about the composition of the rope.
With streaming video, image quality ultimately comes down to the codec and the bitrate. They probably use a relatively low bitrate regardless of codec.
Bitrate, resolution, and codec are all of course critical, and not knowing all three makes it impossible to judge how good or bad it will look. Sadly the resolution is the only one of the three that's easy to describe to consumers, so here we are.
IME Netflix is a close 2nd best after Apple, which I don't think I can distinguish from a 4K BluRay. I've found that the quality depends on the platform a little -- for Netflix the native LG app seems to look best on my LG TV, while Apple looks best on the Apple TV app (perhaps unsurprisingly).
Amazon Prime 4K HDR on the other hand looks like garbage on every platform I've used -- the compression is unbearable in any dark scene.
I would put Disney+ after apple. Both AppleTV+ and Disney+ consistently looks great to me. Netflix is strange as it generally looks good but whatever compression they use does something funny to the picture which makes it look fuzzy and sharp at the same time to me.
Netflix: 15-18 Mbps
Disney+: 25-30 Mbps
Amazon Prime Video: 15-18 Mbps
Apple TV+: 25-40 Mbps
HBO Max: 15-20 Mbps
This is from an LLM but it tallies with what I remember reading. Apple TV is by far the best, followed by Disney+.
Netflix unfortunately seem to use any improvement in compression encoding efficiency to reduce bitrates, rather than improve PQ at the same bitrate. It's definitely got worse over time. I also remember reading that for content they deem more compressible they use a lower bitrate.
I can sort of get that on the lower plans, but its frustrating they won't improve PQ (or at least keep it the same) for the (expensive) 4K plan.
Netflix 4K is some bs in my experience. A 4K file of the same show, pirated, is vastly better quality. Whatever they do to it is just vandalism.