The Continuous Pleasure Issue: I don't have time to track this down further but responding to Don Robertson allowed me to find this, near the section of Cicero that he quoted. This is Cicero relaying a much later argument, but it seems to me that this is one of the pieces in the puzzle of the meaning of PD4 and the issue of "I call you to continuous pleasure". The argument that pain can be managed is not intended to trivialize pain, but to respond to this argument - that the goal of life cannot be happiness if we can lose it:
Since, then, the whole sum of philosophy is directed to ensure living happily, and since men, from a desire of this one thing, have devoted themselves to this study; but different people make happiness of life to consist in different circumstances; you, for instance, place it in pleasure; and, in the same manner you, on the other hand, make all unhappiness to consist in pain: let us consider, in the first place, what sort of thing this happy life of yours is. But you will grant this, I think, that if there is really any such thing as happiness, it ought to be wholly in the power of a wise man to secure it; for, if a happy life can be lost, it cannot be happy. For who can feel confident that a thing will always remain firm and enduring in his case, which is in reality fleeting and perishable? But the man who distrusts the permanence of his good things, must necessarily fear that some day or other, when he has lost them, he will become miserable; and no man can be happy who is in fear about most important matters.