[petsc-users] How to efficiently assemble this matrix?

[Italo Tasso]

I have two matrices: matrix A is 4n x 4n and matrix C is n x n. I want to
set the values of A using the values of C, like this:

A[4i+k][4j+k] = C[i][j]
0 < i < n
0 < j < n
0 < k < 3

How can I do this efficiently?