Replacing the crappy panels that come with the T480 with a better 4K panel is a somewhat popular mod, and it seems that most people opted for the BOE NV140QUM-N53 as the replacement panel.
However it is not the best 4K 14" 16:9 panel, I think that would be the AUO B140ZAN01.2, as according to panelook.com it has better brightness and color gamut while having only slightly worse contrast.
The B140ZAN01.2 is a 10 bit (8bit + frc) panel and i was a bit concerned that it would not work as as far as i know no one had put a 10bit panel in a T480 before.
I had ordered a B140ZAN01.2 from aliexpress a few weeks ago and it arrived today. I have not yet fully installed it but i tested outside the laptop, and i can confirm it works, and despite being extremely bright at 600cd/m² it did not blow the backlight fuse even at full brightness.
I will post an update on the full replacement process when i get around to it.
update:
I have got the new panel installed, The installation process was nearly the same as detailed in this reddit post: https://www.reddit.com/r/thinkpad/comments/jde1a9/the_hackinpad_t481_heavily_modded_t480/
The panel used in the post is a different (worse) version of the one i used and has identical dimensions.
The end result is good and i dont think anyone could tell the panel has been swapped by just looking at it. The only issue is that the panel is offset up by about 1mm, this is due to the pcb at the bottom being taller, i could open it up and remove more of the raised plastic bits on the inside of the lid to get the panel better aligned but the slight misalignment is not that noticeable so i will just leave it be.
The panel itself is quite comparable to the 2560x1440 hdr 500nit panel (AUO B140QAN02.0) in the x1 carbon gen 6, the biggest difference is that the B140ZAN01.2 gets even brighter, color performance and black levels looked very similar.
If you have any questions or would like to know how to do this mod yourself feel free to ask, i left out a lot of details that have already been mentioned in the other T480 4K mod posts.
updated link with more pictures: https://imgur.com/gallery/s9nblnD
While the additional pixel density does not provide much benefit, and infact is pretty bad for battery life. There are other reasons to go for a 4K panel, 4K panels are usually much better than FHD ones, For example the panel i chose has much better brightness (600nit vs 250nit), contrast (1200:1 vs 700:1) and color gamut (74% vs 34% rec 2020) than the stock FHD panel in the T480. But while not as plentiful you can find very good FHD or 2560x1440 panels aswell. So why did i go for 4K then? Because its a lot more fun! And unlike a FHD or 2560x1440 T480, You cant exactly go and buy a 4K T480 anywhere, You have to mod it yourself.