BIG ADVISE FIRST!!! Before watching this season of Tenchi Muyou you have to watch the S3 of this one (duh!), Tenchi Muyo! GXP and Isekai no Seikishi Monogatari. This season ties the 3 of them, in my case, I haven't watched GPX and it hurt my enjoyment a bit.
Second, my view on this 4 seasons of the show, the first season was a solid and enjoyable entry that I really recommend for anyone that likes cute girls, harem and an actual story. Second season is a bit worse than the first one but it's decent too. Third and Fourth are a bit disappointing.
The problem with this show is that it feels stale, there is no progression and the plot becomes less important with every season. The lack of focus in the characters also hurt the harem part and in the last 2 seasons I doubt if you could even call this show a harem because of the lack of romance... and ecchi. This show becomes kinda like a SoL with no focus on the characters, and to shake up the things a bit the keep adding and discarding characters without developing anything.
In terms of animation, the first one has the best one (and it's actually good compared to most of todays shows) and it really made the story more enjoyable. Seasons 2 animation was kinda weak and in s3 and 4 it was fine.
Art, it improves with every season imo.
Overrall, a decent franchise that deserves to be checked out, but the last 2 seasons of the main series are a bit disappointing... but they never reached the point of being "bad".
I decided to made my review in the season because I think that the advise is the most important part of the review, lol.