Depends on the impedance of the 2nd cab. If is an 8 ohm cab, then yes, both combo and 2nd cab plug into the 4 ohm output jacks.
If you look at any of the more recent amp model's user manual on Mesa's website, they began including the pages that had several different combinations of speaker loads and how to make us of them. Some they call a safe miss-match (combination of a 16 ohm speaker and 8 ohm speaker).
For example: If the 2nd cab was 16 ohms, the safe miss-match according to Mesa is to plug that into the 8 ohm jack and the 8 ohm combo speaker plugs into the 4 ohm jack.
I am just stating the basics in general. Tried to look up the Mark IV user manual but Mesa lost their customer support pages, nothing comes up after selecting the manuals button on customer support.