X4 has been really easy as well, except for the final form of the final boss, it's annoying.
I know X1 amazingly well so it's easy. X2 and X3 have a lot of hidden stuff and I forget the order and what's where, so that takes me awhile, but they're not too too difficult. As for X5 and X6, haven't played yet, but we'll see. I think you can change the difficulty level of the later games, I never checked for X4, but I know X5 has it under options

THE JACKEL