That's not actually true, it's just the easiest answer in the eyes of the fanbase. Marshmallow Man and I have argued to hell and back over the finer details of what's going on in Iris's head, and there is really a huge lack of detail in that scenario. While, yes, she is grief-stricken and not right in the head, there's any number of ways to spin that besides merely vengeance. And I don't buy that being the only thing on her mind, if it was at all, given her dialogue at death.
I wouldn't say it's easiest answer, rather, it's the most dramatic. Iris would have witnessed X's internal struggle, whereas with Zero, she would have witnessed cold blood and warrior's pride. The manga takes some liberties with X4's story, but it definitely was
epic to see Zero slice Colonel in two, followed by Iris, who was forced to watch, to be overcome by shock and transforming into the ultimate soldier.
All in all, I think it definitely works to mess with Zero's "I won't hesistate, if an enemy appears, I will destroy it" mentality. For the ideal of X's human - repliroid utopia, Colonel has to be eliminated without hesitation. In turn he is in conflict with the ideal of Iris' Repliroid-only world. Zero is torn between the two people he believes in and is unable to save the girl who couldn't fight on her own without destroying herself.
Of course, even if X was to kill Colonel, it is still the same ideal that results in Iris' inevitable death. "What am I fighting for?", X and Zero just focus on two aspects of this ideal; the kind-hearted side and the cold-blooded side. Dependant on who kills Colonel, Iris' emotions go from straightforward shock and revenge, to torn between the two people she loved to pondering about these ideals and dreams. The girl wants the Repliroid-only world, but the utopia in which humans and repliroids can live together is also beautiful. The people she loves are alike, warriors driven by pride and without hesitation. The man whose ideal is the strongest is like herself, full of hesitation but still forced to fight. The latter is a lot like the relationship between Zero - Ciel - X, but thankfully Ciel and X's ideals align, avoiding a repeat of the Iris tragedy.
For the purpose of all the aforementioned complicated emotions being kept in tact, I would think it'd be best to go with an XS' style scenario of X and Zero vs Colonel and then splitting up to face Double and Iris, as with Berkana and Gareth. This adds an additional layer of depth to the plot, as it was Iris herself that witnessed X and Zero's trust and teamwork firsthand.
The above scenario also mirrors the XS's situation for X and Zero. Berkana and Double were both cases in which X did not hesitate to fight. Whereas Zero was faced with Gareth, who is much more questionable and actually is a lot like Colonel.
However, an XS scenario for X4 really requires a lot of rewriting, the game's cut scenes are character exclusive, most notable being Zero still at Hunter Base after X took care of Sigma. Still, with the way later games refer to the Repliforce incident, it really seems to be most likely.