Every mark in Section II is paid for one specific thing you establish — a correct equation set up, a limit substituted, a contradiction stated. The marker has a list, one line per mark, and ticks what is on your page.
So a 4-mark part is four things. If you wrote three of them and stopped, you have three marks — not zero, not four. And if you wrote all four but one is crossed out, it is not on the page.