You need a lot more information to come up with a full HA and DR plan other than "I have four servers, now what?"

Are you on SQL Server 2016 or 2014 or an earlier version? For the general part of HA, and to a lesser extent DR, I'd look to using Availbility Groups if I were on that version of SQL Server. However that's really only fully functional on 2014 or better (and if you're building this out from scratch, move to 2016).

100% is impossible. 99.999% generally costs millions. You're, at best, looking at four nines 99.99%. That's 52.56 minutes a year for down time and maintenance. That means you have to configure everything nearly perfectly and have minimal issues.

A bunch of other questions present themselves, what is your RTO, Recovery Time Objective, the amount of time it will take to recover in the event of a catastrophic failure? What is your RPO, Recovery Point Objective, the amount of data measured generally in time, you're prepared to lose? These are business discussions you have to have in order to drive the technical discussion.

You've mentioned servers, but where are they? You can't have a DR plan with all the servers in one room. They need to be in different locations. Preferably somewhat far apart locations. This adds latency which can affect both RPO and RTO in the event you're failing over from your primary location to your secondary location.

How about backups? Thought that through yet? You need that as part of your DR plan. Be sure to set up testing for them.

In short, this is an enormous topic and you're not going to get adequate advice from a forum of this nature, especially based on the amount of information you've supplied. I would strongly suggest you get a consultant who specializes in this sort of thing to help. I know a bunch of experts in this area if you'd like some recommendations.