In case MTP mode isn’t working on your Redmi 4X device, and your PC can’t find your device when it’s connected via USB cable, then you might want to try this simple solution. This was tested on MULTIROM firmware, but will probably work on other versions as well.
Solution for not working MTP Mode on Redmi 4X
Open “system/build.prop” and add these lines in the bottom of the file.
Save and confirm changes.
That’s it! MTP mode should now work perfectly fine!
Here you can find more detailed guide (it’s written for Redmi 4 Prime, but works pretty much the same way): Fix MTP mode on Redmi 4 Prime