Most setups I've seen where the plow is attached to the bucket involve bolting thru the bottom of the bucket towards the back. The exception being the "snow pushers" you see in a parking lot that slip over the lip of the bucket & are chained in place.
In your case (4 in 1), the back of the bottom bucket isn't as strong as a regular bucket due to the hinged action, thus your idea to attach to the sides would be a sturdier mount.
I still say upgrade to the quick attach if you can spare the cash - you can retrofit your 4 in 1 bucket with one of the aforementioned plates. Your plow is going to hang waaaay out in front (which is tough if you've got tight quarters or undulating terrain).