1/4 the number of (n+1) X 4 binary arrays with all 2 X 2 subblock sums the same.

Terms

a(0) =9a(1) =11a(2) =14a(3) =20a(4) =30a(5) =50a(6) =86a(7) =158a(8) =294a(9) =566a(10) =1094a(11) =2150a(12) =4230a(13) =8390a(14) =16646a(15) =33158a(16) =66054a(17) =131846a(18) =263174a(19) =525830a(20) =1050630a(21) =2100230a(22) =4198406a(23) =8394758a(24) =16785414a(25) =33566726a(26) =67125254a(27) =134242310a(28) =268468230a(29) =536920070