I know this an older thread but i hope to help others like me.
I am running the DJI Go App solely on android so maybe not everything can be done the same way on iOS but it should be possible.
First off: within the DJI Go App it is possible to download the 4K Video after the flight - you have to start the Video Editor and then select the corresponding video, there is a download to device button (on Android - just appeared in the latest update maybe).
Please be advised: the download can take a while - 5 Minutes of 4K Video took me about 10 Minutes downloading to my Galaxy S8 Phone. If you have the possibility to read the micro sd card directly on your phone, i would advise you to do that if you are short on time and batteries.
I often download the 4K Videos without taking out the sd card because i don't like the hassle and want to do most things wirelessly ;-)
Next up: transcoding or video editing on mobile devices. It is true, that on the usual desktop PC (no matter if linux, windows or macos) the editing of 4K videos takes a long time, but depending solely on the raw CPU-Power (if you are doing software encoding) or special features (like assisted GPU-Encoding). Another possibility is Hardware transcoding (with intel quicksync or other features). Typically software encoding is favored because it will yield the best results - quality-wise. Hardware Encoding can be
alot faster.
But well, i like to do everything on the devices i carry around and that does not include my desktop pc. So then i thought: my Galaxy S8 is capable of recording 4K Videos. That simple fact means: the device has a hardware encoder ready for 4K built in. If your iPad supports 4K Recording, then there will also be a hardware encoder built in. Simple as that.
The presence of a hardware encoder has 2 positive effects if it can be used: encoding video does not use much energy, the chip is tailored specifically for this job. and it's pretty fast. how fast you might ask? well, typically at least in the order 1:1, meaning: 10 minutes of video takes about 10 minutes of encoding - in many cases even alot faster!
If the hardware encoder can be accessed by third party apps on your phone, which is the usual case, you only need an app which does support 4K encoding AND uses a hardware encoder if present on your phone.
I searched through the whole appstore and tested many apps until i found a quite good app (i don't do any advertising here, just sharing my findings ... )
the app is called "kinemaster" and is available on android and iOS. in my opinion it is one of the best video editing apps i have ever seen. It fully supports 4K decoding and encoding on my samsung galaxy s8, even lets me change the bitrate and has tons of features - typically only found on professional video editing software (like adobe premiere): video layering, chroma keys (green screen), effects and so on.
Usually i am just interested in the cutting and re-encoding of the video to 4K, but it's good to know, the app could do more.
The only downside: the app is not really free, without the licence you can use most of the features (at least on android) but there will be a watermark placed on all produced videos. if you choose to pay for it (aroud 30 dollars per year) all features are present and the forced watermarks are gone.
With this setup i can use my DJI Mavic without the need for a desktop pc anymore, i edit and upload all my videos directly on my smartphone - and doing that within no-time
I hope this helps others like me, searching for a similar setup
