three layers price is the same as 4 layers. and we will add a empty layer for it. So you can draw 3 layers, but we would better to use 4 layers, it is easy to layer out

Using all 4 layers is better for manufacturing because it balances out the copper. Havng one layer with no copper may cause the board to warp. Maybe not a problem in a small, one-off prototype but can cause reliability issues in high volume and/or large area PCBs. :)