The LAST thing you need is 4K. (Unless you're intending these videos to run on Netflix)
4K compounds by four times every production problem from principal photography (the shoot) to delivering the show to YouTube. HDTV resolution (2K) is more than enough.
Shooting 4K for YouTube videos is like having a Sprinter that does 150 mph. Fun, maybe. But expensive, difficult to implement and, ultimately, useless.
Consider this: 90% of what you've been seeing in the theatres for all those years is 2K. Only recently have cinema screens displayed more than about 2,000 pixels in the horizontal direction. That's what "2K" is.
OK, gang, listen up. Repeat after me:
"SOUND IS MORE IMPORTANT THAN PICTURE"
Go on, say it out loud. Listen to yourself as you're saying it. It's true. And this from a cinematographer with lots of Hollywood credits. (me) And from someone who's made hundreds of training films and documentaries. Ain't bragging, just sayin'
SOUND IS MORE IMPORTANT THAN PICTURE. If you can't understand what's being said, the whole exercise is a waste of time.
For much of what you're intending, the picture quality from a phone is good enough.
IF you can find a way of holding it steady. There are few things more annoying than shaky footage. One of them is shztty sound.
A tiny camera has one singular advantage over a big one: It can get into tiny places. Like on top of the bell housing where the crankshaft position sensor lives.
That leaves you with enough room under there for the tech doing the work, the camera guy shooting the video and the lighting hardware making it all visible.
A direct connection from the camera to the microphone is nice, but not always necessary. Many training films are shot in what's called "voice over". The words (narration) are recorded separately from the video and installed into the show in the editing process. If you intend the narrator to be speaking on camera (called "sync sound"), the process becomes more difficult, unforgiving, slow, expensive and complex. For that reason, historically most docs and training films use sync sound sparingly.
In that case, you usually DO need a direct connection from mic to camera, either wired or wireless. You also obviously need an input on the camera to accept this connection. Phones lack this. Cheap camcorders frequently do have this vital port. You
can record the on-camera sound separately, but this requires more time and expertise in editing to marry the two media together later.
I have a good phone camera - a Google Pixel 3. It's pretty good, but takes pretty careful handling to keep it steady enough to hand hold. I'm no longer good enough at this, as you'll see in the accompanying video. However last summer I came upon a scenario that just begged me to play with my new toy. So, when it presented itself, I just started shooting. This whole thing was shot in less than an hour, entirely on my phone.
If I'd been using a "proper" camera, I might not have gotten away with this shoot. Usually, as soon as you bring out a camera, everything changes. But in this case, nobody noticed, nobody cared. It was just "some guy with a phone". This is an amazing new freedom for film makers.
Anyway, here it is. This is not great film-making, it was shot just for fun as a camera test, but it actually turned out pretty well, considering the prep time. ie, zero, and the camera budget.
This is a 1K, highly compressed version.
https://www.youtube.com/watch?v=v7uaGFLhbJs&feature=youtu.be