Re: tl;dr: Uninteresting? Kind of, but let's look at the bigger picture.
Yeah, I agree with this! I don't think anyone expects an awesome plot from an old shooting game. I meant something else, I just didn't explain it well.
It's more that, a game is much more fun to play when you have a sidekick or a support character with you. X3 hardly had any plot, but you could call Zero at any time, he shown up to help a few times, and in the ending he stands by your side. And then you have X4, with separate storylines. You just keep facing enemies without anyone to help out or even pat your shoulder. Maybe it's just me, but I was feeling tired fast and the game story felt especially depressing.
Also, I didn't like Zero as lead character in Zero-series. He's either way silent or grumpy, and in Zero4 he was driving me crazy with those "don't thank me, I didn't do it for you" lines. He never shown a good feeling that would make me like him, so I paid no attention to him until I played the X-series. In that way, I'm not sure if Zero is a good lead character either... not like this, anyway.