New Omicron booster is here! How well will it work?

The first COVID booster that targets the Omicron variant has finally been introduced to Australia's vaccine rollout.

The vaccine from Moderna targets the BA.1 Omicron subvariant, but that's no longer the dominant strain.

Omicron has continued to evolve since the vaccine was updated, and the dominant strain is now BA5.

So how well will the new vaccine work with BA5? And if you've already had 4 doses, why can't you get this one as your 5th?