A046270 Largest prime substring in 4^n (0 if none).

0, 0, 0, 0, 5, 2, 409, 163, 6553, 2621, 857, 419, 1677721, 71, 43, 107, 4294967, 171798691, 6871947673, 779069, 511627, 4398046511, 18604441, 703687441, 28147497671, 11258999, 59962737049, 180143985094819, 759403792793