A086474 a(1) = 4 and then least composite such that every partial concatenation of 2 or more terms is a prime.

4, 21, 39, 49, 93, 99, 143, 183, 207, 231, 247, 261, 267, 531, 533, 549, 889, 999, 1167, 1641, 1701, 1809, 2019, 2327, 2349, 2469, 2553, 3043, 3093, 3123, 3159, 3639, 3857, 3937, 4197, 4221, 4233, 4323, 5271, 5369, 5611, 5661, 6641, 7599, 7819, 8021, 8043