Question

when should be taken +4 oxidation number of carbon and when should be taken -4 oxidation number of carbon

Answer 1

When carbon gives away 4 electrons at its outermost orbit you should write +4 and when carbon takes 4 electrons from other element to complete octet you should write -4 but usually carbon shares electron not give or take.

Answer 2

when the carbon loose electron to form a bond then we write +4 .....ND when it gain to complete it's valence she'll than we write -4