I've generally been assuming that the jumps will all be 4 years (so 10-14-18-22).
Since the story always seems to start in/focus on fall, I'm going to assume Step 3 would be the start of college, because most people are 17 at the start of high school senior year and 18 at the start of college freshman year. And having Step 3 be a massive shake-up because you're now going to a completely different school (and Qiu and Tamarack wouldn't necessarily come with you) could make for some interesting conflict dynamics.
Of course, that's all just pure supposition on my part. And I could easily see the argument that Step 3 should be senior year of high school for exactly the same reason - because shifting the focus to college throws a massive monkey wrench into tons of things, and sort of requires a reset of most of the cast. Whereas a story set in senior year means you can keep all your existing cast, and make the anxiety about planning to go to college/everyone splitting up the core of the drama.