$4 /mo per user.
$6/mo per user after 12 months.
You’re right; it’s not very clear from the main page, and instead just gives the pricing tiers you’ve mentioned. But in fact, there is actually a simple answer for this.
There are actually two tiers for Cloud Identity; Free tier, and Premium tier. The pricing you’ve indicated is specifically for the Premium tier, which has some extra features, specifically around security and compliance. There’s a great comparison of the two versions of it, available on the Google Support site: Compare Cloud Identity features and editions
Google definitely haven’t done a great job of advertising that on their front page for the service, where it mentions nothing about the free tier at all.