Ethylene (E) and 4-methyl-1-pentene (4M1P) are copolymerized using an α-diimine Ni(II) catalyst with MAO (methylaluminoxane) or Et₂AlCl (diethylaluminium chloride) as the cocatalyst. A series of copolymers with a 4M1P comonomer content ranging from 0.94 to 36.73 mol% are obtained. A detailed ¹³C NMR assignment is presented and this thorough analysis has opened up the first full description of this interesting family of copolymers. Manifold branched copolymers are obtained with no noticeable differences in the branching distribution by using MAO or Et₂AlCl. The resonance assignments are correlated with the chain-walking mechanism: branching analysis shows that the total amount of 2,1 insertion of the comonomer, followed by backward migration of the nickel active species along the polymer chain, is higher than that of 1,2 inserted 4M1P.