This is my seventh (and probably last) batch of sequence puzzles that are nasty and hard to solve; I stress that each of them has a clear and justifiable solution.
Sequence 1:
RC, NU, RT, RS, PI, ??, AN, PT
Sequence 2:
A, ?, D, ?, G, H, J, K, L
Sequence 3:
?, ?, ?, 1, 7, 0, 7, 1, 7, 3, 4, 8, 9, 3, 4, 1, 2, 4, 2, 5, 9, 1, ?, ?, ...
Sequence 4:
O, ?, ?, H, R, A, U, ?, U, E, ?, ?, ?, ...