How rare is this?

G4 storms are ~100 per solar cycle (~11 years).

So roughly 9 G4 events/year on average.

But they should mostly be in the same part of the cycle rather than spread evenly.

It probably wouldn't make sense to calculate "average snow days per month" across an entire calendar year (in most places...), this is the same thing.

Like 20-25 years rare according to some space weather youtuber.