Welcome back!
I would recommend you run PoTD to re-learn things, unless you want to go through the story again to refresh yourself. They recently added a "Returning Adventurer" status that gives to access to resources like the Novice Network and EXP bonuses. You get a little icon by your name that indicates this too (if you choose to accept it).
They've made a bunch of changes to combat since 4.0 dropped so almost everyone has had to re-learn their classes since then.