What is your concern/confusion? That you will have a 4 ohm load on an 8ohm OT secondary? That was how Fender did it on most of their amps for decades. Their output transformers were very robust and they were not concerned with impedance mismatching. They just added an extra jack in parallel with the speaker jack so you could add another speaker. When you used the ext.spkr. jack you would always end up with an impedance mismatch.