Legendary Campaign Mission 4 no Achievement

I completed up to rally point Charlie on the mission infinity, I decided to have a break so backed out by the save and quit option. Later on after had completed the mission, starting from rally point Charlie I did not receive an achievement for completing the mission and it does not say that i have completed mission 4 on legendary. I have unlocked mission 5 however.
has anyone else had this problem or know how to fix it?

Did you manual select that checkpoint or did you simply hit Continue campaign from the main menu? If you manual selected that checkpoint it should have told you that your save progress will be lost.

ya the same thing happened to my friend idk why they did that but you have to do the whole mission you cant leave and come back at a check point you wont get the achievement so just do that mission again without taking a break and you should get it.