Abstract The cross section and tensor analysing power t20 of the d d → 4 He η reaction have been measured at six c.m. momenta, 10 ⩽ pη ⩽ 90 MeV/c. The threshold value of t20 is consistent with 1 2 , which follows from parity conservation and Bose symmetry. The much slower momentum variation observed for the reaction amplitude, as compared to that for the analogous pd → 3 He η case, strongly suggests the existence of a quasi-bound state in the η4He system and simple optical model fits tentatively indicate that this might also be the case for η3He.