You'd have to disable the headphone jack in order to accomplish this, then wire the output transformers in series (when wired for half the intended load) and parallel the input jacks.
Given that the SEX 4 makes more than twice the power of the SEX 3, we don't really expect there to be a ton of demand for performing this modification, but if you have two SEX 4 amps in front of you, we can walk you through it.