Number of order-4 ribbon tilings for a 4 X n strip.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =60a(5) =160a(6) =455a(7) =1379a(8) =3849a(9) =10811a(10) =30690a(11) =88447a(12) =250980a(13) =712381a(14) =2025309a(15) =5778250a(16) =16437468a(17) =46754562a(18) =133015365a(19) =378725526a(20) =1077736517a(21) =3066752734a(22) =8726710102a(23) =24836946216a(24) =70681078421a(25) =201141628889a(26) =572399433419