You remove one of these counters from the card at the beginning of your upkeep and then cast it without paying its mana cost when the last counter is removed
But despite the emphasis on spells costing four mana or more, I think that the most aggressive versions of UR are the most powerful ones, especially if youre not splashing in other colors
Niv-Mizzet, Parun + Niv-Mizzet, the Firemind Niv-Mizzet, Parun is a top 50 commander that even heads up some Izzet cEDH decks, and stars in Izzet's strongest combo with Curiosity
Also, its not like your entire strategy is tied into filling your graveyard, so the deck isnt going to crumble in the face of graveyard hate thats played early and often
In Modern, you want to play this effect at Instant speed even if you have to settle for +3/+3 in that other format
This makes Belbe a great big mana commander that also offers some defensive measure by incentivizing your opponents to attack each other instead of you