I suppose my hints revealed what I think about the matter.
Plato would reasonably suppose that we judge that something is shameful (ignoble, aischron) for a reason and that--along the lines of the Euthyphro--the reason why we so judge it could not be our being pained when we judge it so. Thus any pain we take when judging something aischron, or pleasure we feel when judging something kalon, could not be that on account of which we judge it to be so.
Plato would also reasonably suppose that a judgment that something is aischron implies a repulsion from the thing so judged; also, that there are two possible grounds for our being repulsed from something, that it is painful or bad, just as there are two possible grounds for our being drawn to something, that it is pleasant or good.
Thus, in the argument:
1. A's wronging B is more shameful than B's being wronged by A.Premises 2. and 3. simply give, plausibly, the general grounds on which someone might judge something to be shameful or more shameful, and premise 4 points out that the basis for this judgment cannot be the pain present in the action/passion, since this predominates in the passion, not the action.
2. What is shameful is so on account of its being painful or bad.
3. What is more shameful is so on account of its being more painful or worse.
4. A's wronging B is not more painful than B's being wronged by A.
5. Thus, A's wronging B is worse than B's being wronged by A.
6. Thus, generally, it is worse to do wrong than to suffer it.
The argument, then, is not invalid; and (as a reader observed) it would irrelevant, for the argument, that Polus is drawn to accept 2. and 3. on bad grounds.
On the other hand, Plato shows considerable artfulness in having Socrates lead Polus to accept 2. and 3. in the way that he does. It is typical in a Platonic dialogue that an argument have points of vulnerability, which are defended against later in the dialogue (the Phaedo being a star example of this). What are the objections that might be raised against the argument as I have construed it?
(1) That our judgments that actions are shameful or noble simply have no basis; they lack a ground. There is nothing on account of which we judge actions to be shameful or noble.Yet these are precisely the objections that Callicles raises, and which Socrates aims to refute in his refutations of Callicles. Callicles effectively urges (1), when he maintains that the judgment that it is more shameful to wrong than to suffer wrong is 'by convention'; and he puts forward (2) insofar as he argues that 'good' amounts to no more than 'pleasant', and 'bad' to 'painful'.
(2) That there is nothing in an action or passion to which we could be drawn or attracted, except the pleasure or pain that they involve.
Polus the character is drawn to Callicles yet also capable of being swayed by Socrates. It is therefore artful of Plato to have Polus accept premises 2. and 3. on grounds which still suggest some allegiance to Callicles.
Yet for all that the argument is not invalid by equivocation, as Vlastos argued.