Step3. Inners at top layer.

Now you made four columns.

You can position inner cubes at top layer using r,l,u,d,f,b, and U, D layers.
If you use only these layers, you never mix your four columns.
If possible, I make a pair of inner cubes in middle layer, then position it.

ex. a->*: dr'd'r or b'rb,       b->*: bDr2b'