I mentioned this in the other thread, but I asked for clarity from Fender on the topic of the 4/8-ohm switch, and this is what I was told:
"The power rating is 400W/8ohms (internal speakers) or 800W with an extension cabinet. The impedance switch allows for some flexibility to pair different cabinets with the combo other than a single 8ohm cabinet. Now a single 4ohm cabinet is another option, or 2-8ohm cabinets daisy chained since there is only 1 extension jack present.
Adding an 8ohm extension cabinet creates a 4ohm total load, with 800W delivered evenly to each cabinet (400W ea).
Adding a 4ohm (total) extension cabinet creates a 2.67ohm total load to the amplifier, and the power would divide unevenly between the cabinets - roughly 300W to the internal speakers, and 500W to the external 4ohm cabinet(s). To keep things simple for the customer, we avoided referring to the 2.67ohms total load on the product and only focused on the rating/impedance of the external cabinet connected.
There are no power resistors or anything complicated involved. The switch simply reduces the output to match the lower impedance for ‘constant’ power. The amp won’t be damaged if the switch is set in the wrong position, but there is risk of activating thermal protection circuits particularly if a 4ohm cabinet is used with the switch set to 8ohm. The amp produces less power in the reverse situation (ext cabinet = 8ohm, switch = 4ohm). The minimum load impedance for this product is 2.67ohm, so connection of external cabinets below 4ohms (total) is not recommended."
That makes sense to me.