Some Thermal Interface Material or TIM's do dry up and/or loose effectiveness after a couple years.
But i would only replace the TIM if you notice that your CPU is running at a hotter temperature then it normally ran at. Also make sure your heat sink has no dust at all lodged in it and make sure your fans are running at the same rpms as before. Fans can loose RPM's when dust gets inside the motor brushes and you also have bearing wear. Also make sure you have the same case ventilation as before.
As for artic MX-4 it should work fine, probably a little better the MX-2. But if you have a tube of MX-2 with some left in it I would just use that instead of purchasing any new TIM...........all the quality TIM's perform within a degree or 2 of each other so it does not matter much what you use. You actually could use some mayonaise or lipstick "the shimmery/glittery the better" in a pinch
EDIT: Yes if it is still in the tube and is not all dried up it will be fine. I would push out 1 match head worth or 2 "2 match heads only if you have enough" of it onto a paper towel. Then use the tube it will work just fine.
It's highly Doubtful but if uyou go to put a match heads worth on a paper towel and you notice it separated and is not mixed anymore then I would buy a new tube, or push out what is left and remix it thoroughly then apply.
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