It depends on the size of the screen, screen fabric, visual acuity, distance to screen, what that individual is used to, quality of the source, environment/room of the setup.
A 1080p disc + 4K Enhancement will not offer that big of an increase vs. the same disc on 1080p mode. On the same projector.
But even at 1080p the 5040UB will look better then your old 1080p projector.
If the 1080p disc is upscaled by a good upscaler (like madvr) and the signal sent is 4K, there will also be some improvement.
A 4K UHD disc on the 5040UB will result in a more increased improvement.
The resolution that the video was shot as is also relevant. Most movies are shot in 2K, with ~80 titles shot in 4K. There is a difference if the setup allows it.
However, I know some will disagree, but resolution is not the most important aspect of the image, contrast and black level are.
Another aspect is HDR, which is usually released in 4K, but not always. HDR is graded/looks different from SDR. Some grading is better, others not so good. The HDR signal also has to go through a tone mapper, which the 5040UB's and many other projectors is not good.
If most of your viewing is done via disc, a HTPC with madvr will be a great addition. However, streaming is becoming more ubiquitous, especially these days. HTPC with madvr can't do streaming. Some users use the UB series from Panasonic, which can do tone mapping, but it's not close to what madvr. It is said to be better then the 5050UB's tone mapping, but that does not say much.
Additionally, there is also the 10GB bandwidth limitation of the 5040UB, that you can read about.