I'm pretty sure Dinorex has all of X4 beaten in the "high pitched" department. Further X5 really bombed on X's theme and the standard boss music, both key pieces in a game's music library (not that X5 didn't hit its high points among Zero, Sigma, and the special bosses). The Zero Virus music was an impressive piece in itself, but really is ill suited for stage music, IMHO. X6's soundtrack I like quite a bit, believe me.
In terms of individual tracks I'd easily concede that X6 and even X5 generally hit higher points than X4 did, but they also hit more low spots as well (ESPECIALLY X5). On the whole I simply found the quality of X4's soundtrack to be more consistent, thus leading to more memorable tracks. For me, personally, X4 had one of my favorite character themes for X in the series, and definitely my favorite for Dr. Light.
I'll easily give you that X4 has X5 beat on the opening themes, cause with X5 X's starts off annoyingly and gets less annoying but not that much better and Zero's is quite forgettable. Here's the thing though. My feelings towards X4's X opening theme are almost the same as my feelings towards MM II GB AirMan's theme. The actual melody is fantastic, but I just do not like the choice of instruments. While it's not Dinorex high pitched, I'll give you that, it's still high pitched enough in the opening and right before the loop were I just find it annoying. There are times that I think it's worse to have a melody you love & hate the instruments then rather the entire song itself be terrible. As for Zero's theme, I think the crazy techno guitar before the loop overdoes it a bit. Speaking of the loop, using the YouTube OST Station as my guide, it loops way too late into the song. It misses out on the softer, better pitched part after the kick ass into. I cannot remember if that is the same as in the game, cause the soundtrack I have for X4 cuts it off after the...2nd crazy techno guitar. If that's the case, then poor loopage on that theme.
You and I have gone back and forth on the boss music for many years now, my dear Shell. This is where I know I'm in the minority, but X5's boss theme is one of my absolute favorites in the whole series. I love the bizarre-ness of the chanters in the opening, the odd techno sounds they chose to use for it, along with awesome guitar work and then the drum finish to start the loop again. X4's starts off high pitched, annoying, and repetitive. Then when that's finally over it's lowers the pitch and by the time they get to actual guitar work at the end, the song loops again. It's short and has so little substance. If I didn't think X3's was worse and if I had ANY clue where they were trying to go with X7's boss music, it'd be my least favorite.
While Zero Rave is ill suited for...everything really, at least it's not f'in boring, which BOTH Final Weapon stage songs are. Not only are they boring, but they are short. Neither of them make it 30 seconds before looping the boredom again. Zero Stage 1 in X5 starts off really well, with the creepiness settling in and the chanters in the background...and then goes off somewhere with it's choice of sounds. And Zero Rave is Zero Rave!
Finally, X4's soundtrack may be more consistent as a WHOLE than X5's (since that soundtrack just really gets weird at times) but I think the consistency of X4's instrument choices is what makes it so sub-par. This is comparing it to not just X5 & X6, but all the X soundtracks. I have to really like, or really hate, most things about a song to find it memorable.
Haha, again this is all just opinion though. I really don't like Bruce Springsteen at all and he's BELOVED!