Your kid has to be vaccinated for mmr in his 18 month but you forgot and gave it in 22 month will it work?

It should be no problem. As with most vaccinations, it is much more important to make sure you wait long enough than it is to wait too long. In other Words, as long as you've waited at least the appropriate amount of time since the last vaccination, the fact that you are "4 months late" shouldn't make any difference at all.