And once you do get the go-ahead from your Arcanist trainer to add extra classes, the NPCs you'll want to talk to are the Receptionists at the guildhalls for each class you want to add. They'll give you a quest (just talking with them about the history of the guild) and then ask again whether you want to join, at which point they'll send you to the guildmaster for your starter tool or weapon.

Where you've read about level 4 crafters, that's because each class (whether crafting or combat) has its own level. You'll be at least level 10 as an Arcanist, but start out at level 1 in each new class you add.