Product_{k>=1} ((1 + x^k)^a(k)) = 1 + 4x.

Terms

a(0) =4a(1) =-6a(2) =20a(3) =-66a(4) =204a(5) =-670a(6) =2340a(7) =-8226a(8) =29120a(9) =-104754a(10) =381300a(11) =-1398410a(12) =5162220a(13) =-19172790a(14) =71582716a(15) =-268439586a(16) =1010580540a(17) =-3817733920a(18) =14467258260a(19) =-54975633702a(20) =209430785460a(21) =-799644629550