a(1) = 4; a(n) = smallest composite number of form k*a(n-1) + 1 with k > 1.

Terms

a(0) =4a(1) =9a(2) =28a(3) =57a(4) =115a(5) =231a(6) =694a(7) =1389a(8) =2779a(9) =5559a(10) =16678a(11) =33357a(12) =66715a(13) =133431a(14) =400294a(15) =800589a(16) =1601179a(17) =3202359a(18) =9607078a(19) =19214157a(20) =38428315a(21) =76856631a(22) =153713263a(23) =307426527a(24) =614853055a(25) =1229706111a(26) =3689118334a(27) =7378236669a(28) =14756473339