Fences and gates are basically governed by the terms of the fencing Act. If the fence separates lot property from common property then the costs are shared 50-50. If they separate two lots, costs are 50-50 between lots and (I think) if the fence separates lot property from non-common property (e.g. it’s an outside fence), the costs belong entirely to the OC (although I would need to check that).
On the question of establishing via a by-law that the the fences are common property, what difference does it make since all four of you have to pay, one way or the other?