I think there is a difference, and it can be worth upgrading if you’re super particular about quality, or if you’re a cinephile who wants access to some of the great new restorations that are now being released exclusively in 4k. The amount of benefit will mainly depend on the display you’re watching on, and title in question.
One thing that doesn’t get mentioned much is many of the new 4k restorations of old films aren’t available on standard blu ray. Often the package will say something like “new 4k transfer approved by the director” but the standard blu ray is just a reprint of previous transfer. So, for an increasing number of movies originally shot on film, the most recent (and often best) transfer can only be viewed on a 4k setup.
As others have mentioned, an added benefit of 4ks is HDR, including the wider color gamut. A good TV (most OLEDs) will be able to take advantage of this added color data and give you an image that is brighter and more dynamic. I think this aspect of the experience is probably more noticeable than the resolution for most home setups. But if you have a TV with mediocre HDR performance, you won’t get much benefit and in fact the colors might end up looking worse than they would just watching in SDR. You’ll see a lot of threads on Reddit complaining about 4k masters being too dark or washed out, and usually this is due to people viewing them on TVs that can’t adequately handle the HDR content.
The resolution improvement won’t be very noticeable unless you have a pretty large TV (65") and are sitting relatively close to it. My girlfriend and I watch a lot of stuff on standard blu rays on her 75" 4k TV, and they look fantastic - no real lack of sharpness. I often forget that I’m not watching 4k content. The difference is more noticeable when I watch things on my 120" projector screen - but even then, the difference is more subtle than what you might expect. I wouldn’t say resolution alone is enough of a reason to drop thousands on a new setup.
