The best place to reach Matt is via his Facebook. There's also
a Facebook group for his students, where you can find a LOT of help, from them as well as Matt.
But as you've seen with a couple of the posts above, any questions you have for Matt, we can answer here! In the case of this group of chords -- some arrangement of C, G (or often G7), Am, and F -- once you can play those, and switch comfortably between them, look up something like "I'm Yours ukulele chords" and you'll find a TON of examples. Pick one, and start to strum.
Matt's point is that, with practice, you can figure them out by ear...but no doubt, the Google method is a nifty shortcut.
If you want to really, really drill down into this, you can check out a YT channel called
Four Chord Simple that has almost 200 tutorials using these four chords, and in plenty of cases, just 2 or 3 of them.
In addition to the fantastic Axis of Awesome video above, this was the first I saw that took it up, a 15-ish year old named Lindsey whose "4 chords 15 Songs video" (including all three that you mentioned,
@bunnyo!) has garnered 3.5 million views!!!! Yeah, she was a kid at the time (6 years ago -- for all I know, she's married with kids of her own now; what do I know? LOL), but she put these together really well. When she followed Let It Be with Let It Go, I nearly fell out of my chair! Genius!
Ricky Somborn did an updated list last year, and his YT channel has a bunch of shorts that also have THREE chord songs. Very beginner-friendly stuff, but fully legit playing and singing. You'll find a LOT of beginner-friendly stuff here, including lots of contemporary stuff. I'm working on his playalong for the Miley Cyrus tune "Flowers" right now myself, and there's plenty more besides.
Lots of us here love Matt (including me! And I first heard about him here in the forum!), and for sure, keep watching his Kanikapila videos where you can put this music theory into action (I never miss them, and frequently rewatch them myself), but trust me, we've got plenty of what you're looking for right here!
Enjoy!
Tim