Reserved List. For reference, here's a list from Scryfall of the non-reserved cards from most to least expensive:
It's safe to say that Grim Tutor (#2) in unplayable in the current metagame (Dark Petition being a reasonable upgrade) despite having prior pedigree, so the most expensive 4-of a player could reasonably want to play is Mox Opal (#8), which happens to be MODERN playable. The next I'd say is Jace, the Mind Sculptor (#10), which may not be a 4-of ever but still gets played in multiples. It's an easy proxy rule that is based on an easily verified information (the reserved list) that doesn't fluctuate (price) or corner players who need "one more proxy" (fixed quantity).