Hi, Ran4.
If you can see the cached images in the 'editor' part of the DJIGo4 app, they're definitely there somewhere... The default location is is Internal storage > DJI > dji.go.v4, but this may have been changed. In the 'General Settings' on the app there is a slider marked 'Cache to SD card'. Whether or not this is active, the text underneath shows the current cache location.
I can see all my cached images in my phone's 'Gallery' app, which is quicker to launch than the DJI one, although you may need to change settings to make it look in the right folders.
The cache only holds thumbnail JPEGS and lower-resolution video, as sent to your phone during flight - complete with any glitches. To transfer the HD images to a phone, you can download via the app while the Spark is connected; to transfer them to a PC either remove the SD card from the aircraft or connect via USB. I prefer the latter, but note that the Spark must have been turned on and given the beeps of joy before you plug the usb lead in. The truly cautious would remove the props.
HTH, post back if not.
Nik