Sammā vācā: Right speech
Speech is weighed by whether it is true, helpful, and rightly timed. "I wrote this very thing to you" is a decision about timing: hard words sent ahead in a letter give the reader room to sit with them alone, where the same words spoken across a room would only put everyone on the defensive. The content stayed sharp, and the delivery was chosen so it could do its work before he arrived.
Domanassa: Sorrow in the mind
Grief of this kind is mental pain rather than bodily. Walking through the door and meeting sadness from the very people he expected to be glad with is the ache he is steering around. So the sorrow goes ahead in an envelope, and by the time he stands in front of them it has already been felt and finished.
Muditā: Gladness at another's good
Gladness rises here from someone else's flourishing. "Them of whom I ought to rejoice" makes joy sound like a debt owed: these are people whose growth is his own happiness, so their trouble reaches him through the same open channel. Love that can be gladdened by them can be wounded by them, the way a parent's mood follows a child's.
Saddhā: Confident trust
Trust of this sort is placed in a person before the outcome is known. "Having confidence in you all" is him staking something on the Corinthians while the letter is still in transit and their reaction unread. Extending that trust does part of the correcting on its own, since people handed a good opinion of themselves tend to rise to meet it.
Kalyāṇamittatā: Friendship in what is good
Such friendship carries a person toward what is wholesome. Joy "shared by all of you" is its mark: gladness that belongs to the whole gathering, with no one left standing outside it while the rest celebrate. A friend like this risks the awkward letter, because keeping the peace by keeping quiet would cost more than the discomfort of writing.