Hmm... (yes, still talking WoW...)
Perhaps that's why they made the change to Focus Fire that they did. Since you want to wait until you have 5 (or at least 4 Frenzy stacks) but it's an instant cast (and doesn't appear to be on the GCD), the only solution then might be a 3rd button like Lyonheart suggests, since it will otherwise hang a macro (it will cast even if you don't have Frenzy stacks, so you just wind up spamming it instead of moving to the next spell).
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