Tetranacci analog of A055502.

Terms

a(0) =0a(1) =2a(2) =3a(3) =7a(4) =13a(5) =29a(6) =53a(7) =103a(8) =199a(9) =389a(10) =751a(11) =1447a(12) =2789a(13) =5381a(14) =10369a(15) =19991a(16) =38543a(17) =74287a(18) =143197a(19) =276019a(20) =532061a(21) =1025579a(22) =1976857a(23) =3810517a(24) =7345031a(25) =14158009a(26) =27290429a(27) =52604017a(28) =101397487a(29) =195449957