So refers back directly to verses 9-10, but also more generally to all the encouragement Paul has been giving his readers throughout the chapter, just as so then in 4.18 refers back to the entire section 4.13-17.

One another … one another. No doubt for stylistic reasons, Paul uses two different expressions in Greek, but they have the same meaning, and therefore need not be distinguished in translation. Help is a literal equivalent for Paul’s metaphorical “build one another up” (cf. Jerusalem Bible “keep on strengthening one another”). Both verbs, encourage and help, suggest continued action over a period of time, and this is made explicit by the following words, just as you are now doing (cf. 4.10). This clause may be rendered as “that is, of course, just what you are now doing,” or “you are, of course, doing just that.”

Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on Paul’s First Letter to the Thessalonians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .