Although many say their isn't much difference between the 3rd and 4th generation, there must be some reasons Leica updated the lens, they don't do it just for fun 😉 and going from 6 to 7 elements is not cost cutting.
Leica lens tester Erwin Puts mentions improved full aperture performance, higher contrast and better correction for field curvature. In actual use you may not see any difference. The larger aperture ring is nice for handling and the squared hood design helps with flare and over-all compactness, and it does take 39mm filters (3rd version series 7 in the hood).
It may not be much better than the 3rd version optically, but one advantage is that if you find a good deal on one its an easy re-sale if it comes to that, and you may even make a bit - the lens does have a following for some reason, THE Leica lens for look and handling for many :angel: