Rx 480 Mining only on 1 card

Hi, I have a rig with 4 x Rx480.

When i use ethminer i use only 1 card or i get only 20 MHS....

I try with ubuntu 14.04 and 16.04 and amdgpu-pro 16.30 and 16.40 and i get the same result.

Can you help me.

Thanks