212 HD 130 Speaker impedance switch

On the rear panel of a Music Man 212HD 130 there are two speaker jacks, the panel reads that these are connected in series. There is also a switch for speaker impedance with a 4 ohm and 8 ohm setting.
I am reasoning that the two speakers contained in this unit are 8 ohms each and they appear to be wired parallel. Thus the load of the two speakers is 4 ohms. Therefore, with the internal speakers plugged in, I keep the impedance switch in the 4 ohm position. Is this correct? Also I assume that since the panel says the jacks are connected in series that if I chose to add an external speaker cabinet I should chose one with a 4ohm load. I would plug the external cabinet into the 2nd speaker jack and keep the internal speakers plugged in. Than I would have two 4 ohm loads in series for a total of 8 ohms. Thus I would switch the speaker impedance to 8 ohms. Is this correct?

If the total impedance of internal speakers is 4 Ohm and the external speakers impedance is 4 Ohms, yes you're right, you've got to sum their impedances.