The AF and tilt screen are the biggest upgrades. The sensor is really good. But then, the sensor in the E3/T2/Pro2 is also really good. Another big difference between the two is the X-E4 does not have the rear scroll wheel. If you use that a lot in your shooting, the X-E3 will be the better camera for you to go with.
Don't forget about the grip that's been removed; the X-E3 handles well enough even with (slightly) bigger lenses - I doubt that the X-E4 will do equally well.
It's a different enough camera that I think it'll appeal to a different type of shooter - and I'm not one of them. That said, it's an intriguing offer, especially when combined with its 27mm II companion - small, elegant and capable.
I'll stick with the X-E3 and the original 27mm - I don't profit from the WR enough, owning an unsealed body, and just switching to get the aperture ring seems frivolous.
I should probably see this as kind of a relief - as much as I appreciate what Fujifilm bring to the table, it's most probably not made for me. However, I enjoy using the X-E3 enough to keep it until I can truely replace the system, mostly the primes - which isn't the case yet. Somewhat irritatingly, the only lens I've so far found a match for is also the most impressive I own for the system, the 90mm f/2 (the Nikon Z 85mm f/1.8 matches its performance when used on the Z 50). The other primes are still really good, small and reasonably priced; the 18-55mm zoom remains unique as well (even though the Z 16-50mm DX gives it a run for its money in terms of IQ in many ways - but build, handling and speed are still way beyond what the little plastic Nikon lens offers).
The X-E4 probably represents a sensible next step for Fujifilm (I think we've seen the last of the A and M lines - this is the new entry point, I guess). I, however, would (only) fall for a weather-sealed version of the X-S10 ... But I think that's not to be. It would, however, completely change the game and make life even more difficult not only for Nikon and Canon, but also for Sony APS-C. Such a camera might actually do even more: Kill the OM-D legacy altogether ... Maybe it's just as well that it won't happen.
M.