I mean, two nines is 3 and a half days of downtime. That's not hard to get as long as I don't make a habit of breaking shit in ways that take a long time to find out, the hardware is decent and I don't disappear for days very often. I'd add that the network should be decent too, but if it's not, my stuff should be fine.
A competent person can dig around and fix most anything good enough in three days and that leaves you 12+ hours for other outages.
Three nines is almost 9 hours a year, but planes almost universally have wifi now, even if it's not great. If you have a bad year, with two big incidents when you're on a plane, you might not make your goal.
At four nines, it's about an hour. Good luck with that one. You're kind of screwed either way. Someone with intimitate knowledge of the system can probably handle an incident or three in time, but it's an enormous amount of work to get people up to speed for that and you can't have very many live training sessions because you don't have the outage budget. At the same time, if you page me and I'm at the cinema, I'm not going to fix it in time.
But, the biggest thing is working to make failure as graceful as possible. As much as possible, each component should be able to fail individually, without affecting the rest of the system. And, when there is an outage of a critical dependency, you need a way to push that outage status upstream and a method to manage load when it comes back up. (Those knobs should be stable and can certainly be documented for others).