99.99% availability is almost always predicated on 24/7 uptime numbers, not 24/6 plus an extra 22 hours.
If you say to someone "We have four nines of uptime", and they ask how you apply kernel patches, and you say "oh we just do it during our 1% of downtime", they'll boggle at you, and for good reason.
If you say to someone "We have four nines of uptime", and they ask how you apply kernel patches, and you say "oh we just do it during our 1% of downtime", they'll boggle at you, and for good reason.