Repeating a repeated sequence

Or, simpler (assuming you mean a vector, not an array)

rep(rep(1:4,each=3),3)

You can do it with a single rep call. The each and times parameters are evaluated sequentially with the each being done first.

rep(1:4, times=3, each=3)  # 'each' done first regardless of order of named parameters
#[1] 1 1 1 2 2 2 3 3 3 4 4 4 1 1 1 2 2 2 3 3 3 4 4 4 1 1 1 2 2 2 3 3 3 4 4 4