TNG's season 4 wins, for me, followed by DS9's season 4.
TAS, though a great piece of Trek, certainly a "Lost Trek" for me, its not as great a season as TNG and DS9 are. Its still, though, a surprisingly good season - episodes with Tribbles, Orion Pirates, time traveling Spock and "Albatross", and quite a few more, all constitute great fun and great joy, especially for this TOS fan, who learned of TAS just one year and a half ago. I'd say its on-par with season 3 - while not having the classics of S3, it didn't have its stinkers, either.
TNG's S4, on the other hand, is probably TNG's finest hour. With a terrific conclusion to the Borg two-parter, we move deeper into the crew's mind, hearts, and all that. This season does what season 7 tried to re-do, in obvious failure - capture the sense of family that has formed between these people. Even a passing Tasha Yar reference doesn't go by without noting how she was still, in mind, part of that family. Not to mention the loads of fun that accompanied this season. I'd say this season finally gave TNG its seperate identity from TOS (one that it should've earned in season 2, but anyway), simply it had the opportunity, to take more chances. And TNG, in this season, took those chances.
DS9 saw Sisko going bald, and Worf coming to the station. How lovely. In the end, I'd say this is a terrific season, equal to season 3 by all means, if not better. However... The overal emphasis on the Klingons of the show was, well, overwhelming. Not bad, no, mind you. I just felt that the show tried to buy some time before really engaging with the Dominion later on. That said, its a brilliant season, with one of my all time favorites - "The Visitor", of course. DS9, as I said, was always a show that improved over the years. The first three seasons were fantastic and all, and will always have a special place in my heart as the introductory Trek, for me. However, DS9 truly became excellent, with Worf's arrival... I'd like to think that Worf wasn't necessarily the factor to that happenstance, but still a contribution. Anyway, DS9's season 4, is a good one.
VOY's season 4 is, for me, quite fun, if part of the brain is shut down from that little voice thats called "reason". 7 of 9 comes aboard, and the show takes a sexier direction that I still find, somewhat, stupid. It seemed to me that 7 of 9 was brought in to just boost the ratings, when they should've just focused on the writing of the said show.
ENT"s season 4 is when the show, for me, became what it should've been in the first place. Though the TOS/TNG reference goes a little bit too much to my stomach, I still feel the characters became more rounded, more real in the process. Not to mention the actual effort on paying attention to established TOS early-Federation history, and not just working things along as they went on. Its only sad that the show had to end with a whimper of a finale, like "These Are The Voayges", an episode whose sole excuse is to see Ent-D again and provide with the useless gimmick of the voiceover "tribute". ENT was never a good show, but ENT's season 4 came the closest to that, IMO.
Overall, here's how I'd rank them:
1) TNG
2) DS9
3) TAS
(gap)
4) ENT
(gigantic gap)
5) VOY