You could equate it (roughly) to 5+ or 6+ on a d6. Or 4+ on a d4, but I would stick with the d6.
I am doing this with some spellcasters. Instead of choosing four spells of a given level, I am just choosing one spell to recharge on 5+, or two to recharge on 6+. Particularly higher spells might be one only. In all, it means that they will cast less spells than they could otherwise, but if they are only alive for a single battle, it seems like a much easier method!