I am not sure about Maths. Assuming it is modular, you would probably have to retake some of the modules you sat this year. And if you struggled with C1/C2, you may find C3/C4 incredibly difficult, more than just a challenge. There is no shame in doing 3 A-levels, and in most cases, having 4 won't really make a difference to a university as they, more often than not, just count the top 3 grades.