Swapping rooks in a 4x4 board

You have a 4x4 chessboard with four black rooks on the top and four white rooks in the bottom.

Your goal is to swap these rooks using the minimum number of steps. It does not matter which rook is which, as long as there are four white rooks on the top and four black rooks in the bottom.

Chess rules apply: rooks can move any number of squares, horizontally (left and right) or vertically (up and down), as long as there is not another piece on the way. White starts. You must alternate black and white moves.