The Satellite send is line level and the slave out is line level and more (depending on how you set the slave level). Neither of these are appropriate for plugging directly into the input of another guitar amp.
If your amp has an effects loop, you can plug these into the effects return (sometimes called power amp input on the older amps)
By they way, are you using the effects loop on your Mark IV? If so, use the Satellite send to go the effects return of another amp. However, if you are not using the effects loop on the Mark IV, you can use the effects send (or slave out) to go to the effects return of the other amp.
Does anyone know if the Satellite send works if you are not using the effects loop on a Mark IV? (Does it acts like 2nd effects send?) Or do you have to have something plugged into the adjacent effects return to have a signal come out of the satellite send?