Who uses these IR capabilities? And at mach 4, the target should already be acquired.... If you're going mach 4 and need to change your detonation point, there are bigger problems you need to address....
Well, there are many missiles that use IR homing. Along with the MANPADS type weapons Gremlin mentioned, all the current iterations of short range air to air missile, for example (AIM-9X, ASRAAM, IRIS-T, one of the Mica variants, the Vympel R-73), use IR homing.
The C variant of the JSOW, intended to engage moving targets, uses IR homing for terminal guidance, and that's a relatively recent development. Several anti-ship missiles use IR for terminal guidance as well, among them the Kongsberg NSM, which is another recent development.
I'm surprised you aren't aware of this as I thought it was common knowledge that IR guidance was a relatively common method of guiding a weapon onto a target. Or are you distinguishing between IR guidance in general and IIR, or imaging infra-red? If that's the case then my mistake, and I'm not sufficiently aware of the differences between various IR guidance methods to really comment.
I'm not sure I understand your second statement. A missile like ASRAAM accelerates to Mach 4 (at least that's the number I hear thrown around, but I am not a military man and so rely on open source information) and is guided onto the target via an IR sensor. As it is intended to engage high performance fighter aircraft, of course there is a need to constantly update the point at which the missile will detonate, based on the movement of the target. But maybe I misunderstood something you said?
Happy to stand corrected on any of the above, as I said maybe I misunderstood you. But I don't know why modern missiles, and even missiles still in development, would be employing IR seekers if they were ineffective at guiding a weapon onto a target, particularly as some weapons are intended to engage high performance aerial targets.