2 caps: 1 for things that are charged for existing (e.g. S3 storage, RDS, EBS, EC2 instances) and 1 for things that are charged when you use them (e.g. bandwidth, lambda, S3 requests). Fail to create new things (e.g. S3 uploads) when the first cap is met.
Why does it use version 4 instead of version 8? Version 4 implies that it's random bits, but it's actually not random. Version 8 doesn't imply anything about what the bits mean.