Catalysis of the reaction: (E)-3,7,11,15-tetramethylhexadec-2-en-1-yl diphosphate + 3 NADP = 2-trans,6-trans,10-trans-geranylgeranyl diphosphate + 3 NADPH + 3 H+.
geranylgeranyl diphosphate reductase activity