A microscopic study of the /sup 2/H(d,..gamma..)/sup 4/He reaction with deuteron and /sup 4/He bound state wave functions which include the D-state components is presented. Cross section and tensor analyzing powers are calculated for deuteron energies between 1 and 15 MeV. Deuteron D-state effects are found to be particularly important in the transition amplitude arising from the /sup 5/S/sub 2/ scattering state. A detailed comparison between cluster and microscopic calculations is presented. Using an effective two-body model the analysis of A/sub y//sub y/ data, in the energy range 1less than or equal toE/sub d/less than or equal to15 MeV, gives for the asymptotic DS state ratio in /sup 4/He the value rho = -0.18.