Fusion+ was never well explained in the core book, other than passing references to it building the Imperium. Likewise, the core mechanics of T4, while a workable system, did not work as advertised. It felt nothing like CT to me, and even less like MT... and the editing quality would have gotten a huge pile of failing grades in any of my freshman college courses.
It was, in a nutshell, a passable system with a few gems and some real stinkers, and didn't play at all like anything else Traveller labelled. It was, for the first time in my life, sufficient to make me wonder what color the sky was in Marc's world, cause it sure didn't seem to be any shade of blue...
I was hopeful, the blind hop of the media blitzed, when T4 came out. (Tho' I was disappointed at the time that SJ (of SJG) didn't get his bid to do MT2E....) We played it. We didn't like the stat-bloat. We tried the This Is Hard rule. It didn't help. Due to stat bloat, it also made PC's ubermenschen in combat. (Skill 3 was sufficient for all but sniping. The skill 6 sniper with Dex F, however, had trouble hitting at range...)
Further, the method of dealing with armor felt like a step backwards after MT or TNE...
And it lacked the promised simplicity. If it had not been hyped the way it had, it might have fared a lot better. But between the hype, and the high quality of the other editions, it had a huge bar to meet, and failed by a long shot.
Lots of good ideas, lost to poor editing, lack of adequate in-depth playtesting, and excessive hype.