From the enumeration of involutions avoiding the pattern 4321.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =2a(6) =4a(7) =6a(8) =15a(9) =31a(10) =67a(11) =155a(12) =343a(13) =787a(14) =1829a(15) =4233a(16) =9940a(17) =23454a(18) =55540a(19) =132390a(20) =316704a(21) =760462a(22) =1833050a(23) =4431552a(24) =10745282a(25) =26125316a(26) =63672044a(27) =155536070a(28) =380739734a(29) =933832952