Are you not liking that it takes up a lot of space on your drive(s) or that your computer can't play back the files?
You could record to one of the recorders that does 4k (Atomos makes some). Or just transcode what you shoot to ProRes.
I guess I should have just given the short answer, though I don't think the 5d4 will produce 4k from the hdmi connection.
Anyway to the op.
I don't know what you shoot, your workflow, do you grade, your editorial system, how you deliver (what type of conforms).
Anyway, the motion jpeg codec is a good codec as it's like all intra where every frame is a single frame and has a high bit rate of 500 to 600mbs. it's also 4:2:2 and though 8 bit it's deep for 8 bit and Canon color is pretty good out of the box. The downside is it's produces large files, needs large storage.
Compared to h264 and avchd (mpg4) cameras, like the sony A7sII which has 8 bit 4:2:0 and 100 mbs, there is a big difference when grading and I've found the mjpeg format grades pretty well.
In regards how to speed up your editing with the mjpeg codec, you're better off transcoding to prores, (if that is your system's native codec) as it puts much less strain on your system and given you only have 1 gig of vram in your system.
You can transcode through many software suites and that's a drag except your editorial suite is going to transcode anyway, whether you do it after the edit or in the background during the edit.
If you want prores out of the camera, it's black magic, or with a license Arri or Sony F5 but of course these cameras don't autofocus. Most film makers don't like autofocus but I believe a lot of that is the cameras.
In my experience the only cameras I've used that track in motion is the Canons . . . d70/80, 1dxII, Mark IV, c300II with their dual pixel pdaf systems.
In regards to prores, Apple has discontinued support QT/prores on windows/pc's and I assume because so many editors moved from fcp7 to premier rather than fcpX and if your doing Premier, a Dell or HP Box is much more useable than the 2010 Apple Flower Vase.
Apple kind of lives by the rule of if you can't beat them, ban them.
In fact there was rumors that Apple would kill off Quicktime at least a useable update of QT7 Pro, since the latest update was for snow leopard, though I use it all the way through Yosemite and they say it will run in Sierra except Apple no longer offers a pro key to unlock 7 to 7 pro.
I firmly believe the only reason Apple continued with prores is their are so many companies with a prores workflow that Apple would receive a lot of heat if they killed it.
Sorry to go off topic, but Apple does what they want, the moment they want. Buy a new powerbook pro and try to find a way to connect a fw800 drive on it.
So if you go prores as a standard for editing, keep an older computer nearby with some empty drives that work with your system so you can copy data over and find someone with a QT7 Pro registration key.
IMO
BC