5D MK IV - Video Settings?

I'm just starting to experiment with video on my 5D Mark 4. Which video compression quality and speed is best to use? I don't plan on shooting 4K, only 1080 HD. I usually shoot in aperture priority mode and have fast cards in both slots. I also have an external microphone too. I saw an on line video that suggested I set audio levels to manual and adjust as needed. It also said set mic gain to +20 when recording to lower electronic noise. I'd like to get this under control by Christmas to get some good video of the Grandkids. Any help is appreciated.

Can't help, but I'm in much the same boat. I've had all the 5D series, currently have the MK4, but have never seriously played with the video, but now it seems it's so much better, I'm keen to give it a go, so will watch this thread.

Unfortunately the answers to all these questions is "It depends. On you."

Higher quality gives, well, higher quality, but chews through memory cards like there's no tomorrow.
Manual sound level setting is more consistent, but requires that you pay attention.

I'd say that exactly which setting you use is probably less important, compared to how you make your film. Think about the good, old principle of letting the subjects do the moving and keeping the camera still is usually better. Edit your movies and remove things that aren't good. If you don't like them too much, other viewers will hate them.

I think the OP misread about the mic levels. What he read was - 20 DB not +. By keeping the average levels at -20 DB, especially when recording music, it gives you sufficient headroom to accommodate the dynamic transients which can be 20 to 25 DB higher. The goal is to prevent these transients peaks from becoming distorted. You generally want these peaks to be at around zero on the scale and no more than a few db above.

Here is a simplified definition of a transient from the internet.

"In acoustics and audio, a transient is a high amplitude, short-duration sound at the beginning of a waveform that occurs in phenomena such as musical sounds, noises or speech."

I would suggest until you learn how to record sound properly that you use the auto feature of the camera's sound mixer.