Concatenations C1 and C2 and C3 and C4 are all prime (see the comment lines).

Terms

a(0) =51a(1) =63a(2) =123a(3) =597a(4) =687a(5) =987a(6) =4429a(7) =12237a(8) =18291a(9) =20013a(10) =29953a(11) =31863a(12) =34653a(13) =48023a(14) =52703a(15) =57147a(16) =59337a(17) =62097a(18) =66727a(19) =71733a(20) =81263a(21) =86589a(22) =101211a(23) =105501a(24) =118617a(25) =121397a(26) =134013a(27) =146191a(28) =147151a(29) =155937