Who has the right of way in a 4-stop sign "intersection" that isn't labelled as "all way"?

"The main road gets priority" applies when the side roads have stop signs and the main road doesn't. There is no rule in Ontario about relative road sizes in the absence of signs. (Driveways and roads, yes, but not two roads.) The Drivers Handbook is online: click "Safe and Responsible Driving " then "Driving through intersections" and finally "Controlled intersections" for all the stop sign details. Main roads are mentioned only in describing a "don't block the intersection in heavy traffic" situation.

That said, your puzzle here is not a puzzle at all. Driver 1 arrives, stops briefly. There is nobody to yield to so Driver 1 continues after stopping. Driver 2 arrives. Either 3 and 4 are nowhere in sight (you haven't specified how closely they all arrive) or they are stopping at their signs still. Driver 2 turns right. Driver 3 may have to wait for Driver 1 to clear the intersection. It's possible Driver 4 will head through while 3 is waiting, though this is technically wrong. Once Driver 1 is out of the way, Driver 3 will turn left, possibly with Driver 4 still stopped and waiting. Then Driver 4, if they haven't gone already, will proceed.

At no point will the main-ness of the streets be relevant. The only iffy bit is that Driver 4 may take advantage of Driver 1 causing Driver 3 to wait, and go "early". That doesn't seem to be dangerous though.

You yield to the person on your right.

If it's busy enough for everyone to arrive at the same time, somebody has probably just gone. That is the spark plug/tiebreaker. Whoever had that person most immediately on their right, goes next.

So if 3 had just gone, 1 is up. Then 2 goes. Then 4, then 3 again.

When you have a spread out intersection like this, sometimes someone goes, and someone else realizes they can move with no conflict, so they go immediately. Note that 2 can move almost the instant 1 goes, since no route of 1 can possibly conflict. 4 has to worry about 1 making an unsignaled left turn onto 2's road, but can proceed the instant 1 and 2's intentions are clear. 3 has to wait until 1 clears its road.

If things get confusing.... Then you fall back on everyone's sense of fair play. For instance, if you are the next 4 and as you sidle up to the stop sign, you plainly see that 3 has been sitting here "awhile", you surely know they go before you...