A094559/4.

Terms

a(0) =0a(1) =0a(2) =1a(3) =4a(4) =19a(5) =76a(6) =316a(7) =1264a(8) =5101a(9) =20404a(10) =81796a(11) =327184a(12) =1309444a(13) =5237776a(14) =20953936a(15) =83815744a(16) =335274259a(17) =1341097036a(18) =5364433276a(19) =21457733104a(20) =85831112764a(21) =343324451056