Let me start off by saying that while there are obviously no guarantees regarding the length of the Twins' postseason run, the team has to be prudent and plan for the possibility of 22 games.
Only three starters are needed for round one of the postseason. A fourth starter will need to be rostered for the second round but may not be needed until the third round. Barring injury it's unlikely that a fifth starter would be needed, and if a five-man rotation were to be used it would probably not be implemented until late in the third round at the soonest. So one of Maeda, Ober, and Ryan needs to be designated as a reliever for the first two rounds and probably beyond. Of those three I think Maeda is unquestionably the best suited to pitch in relief. I also think the team would get the most value from those three by having Maeda available to pitch in relief, especially in the first two rounds. That makes Ryan the third starter and Ober the fourth, with Maeda a reliever (and possible fifth starter late in the postseason).
I think it's also worth pointing out that we are fortunate to have five pitchers who appear capable of doing well as starters in the postseason, with Varland and Keuchel as emergency fallback options.