The Russian T-90 tank uses infrared emitters as countermeasures, looks like a pair of angry eyes: https://www.army-technology.com/wp-content/uploads/sites/3/2...

The cost difference is unlikely to be relevant for a Mach 4, air to air missile. But, if this is small and cheap enough it might get added to a lot of currently unguided weapons like bombs and RPG’s etc.

Militaries constantly make cost vs effectiveness trade offs, and new technology can very easily change the equation.

You put a filter over the front that blocks your charging wavelength, so it's only used internally.

Finally, night vision goggles that look like something out of the movies