10 December, 2019 update: Munki 4 is now an official (not pre-) release, so the below instructions are for when the next version is in prerelease…

25 November, 2019 (original post)
As of this writing, Munki 4 is in the late stages of testing (its current on release candidate 1), so if you want to be part of the testing process and distribute it to part of your fleet, you want to start by first importing it into your Munki repo.

There is a new AutoPkg recipe for this called munkitools4.munki.recipe

First, make the override for it:

autopkg make-override munkitools4.munki

Then, edit the override, so it includes prereleases by adding in the appropriate true to the override’s XML:

<key>Input</key>
<dict>
<key>INCLUDE_PRERELEASES</key>
<string>true</string>

Then, run it as you normally would, just to verify it works (before including it in your scheduled runs):

autopkg run -v munkitools4.munki MakeCatalogs.munki