If the 4x12 cabinet is 8ohms or 16ohms, then your ok to plug into combo and use the roadsters speakers as well because the internal speaker load is 8ohm. If the 4x12 cabinet is 8ohms, then plug it into 4ohm jack and plug internal speaker jack to the other 4ohm jack. If the 4x12 cabinet is 16ohms, then plug that into the 8ohm jack and the internal speaker jack to the 4ohm jack. If just using the internal speakers, then leave the internal speaker jack on 8ohms.
