Sometimes I like to use one frame from a 4K video as a still and need good detail.  However I've found that if the drone was moving while taking the video, the fine detail when you look closely at the still frame is blurred because the shutter speed was to slow (like 1/50).  I think I need a minimum shutter speed of more like 1/120.  On most digital cameras they have a Shutter priority manual mode.  What is the priority on the DJI Go Manual Camera settings and can you change it to shutter priority or is there 3rd party software you can use to do that if you can't with DJI Go?

I don't think there are any third party software programs that support this. I know my P4P does have this function though in Go, unfortunately the P4 does not have it.. The only thing I can suggest is to manually adjust your ISO to provide proper exposure for the locked shutter speed you want. :/  Sorry

Well I am using a P3 but I thought the settings were the same in the GO app, just select manual settings and choose the shutter speed you want, you may need to select an appropriate frame rate and ND filter to facilitate the speed you want in the conditions you are flying.
Does the P4 not allow this?

Optimal exposures for Video and Stills are different. In Video the motion blur is desirable to make it smooth.

What FPS are you shooting the 4k video in? As said above ND filter and select manual settings and then adjust ISO to suit.
You could try 1080@60fps and grab a still from that, and then see if that quality will suit your needs.

Your 4K screen capture will never equal that of a still capture.  You'll have to turn off the video camera and shoot stills if you want any sort of quality.  Can't do both at the same time.

Imagine the video file size if each frame was captured in still mode.  Files would be in the terabyte range quickly.  E.g. 29 RAW frames at 17,800 KB would equal one GB in two seconds of video.  Even 29 frames in one second of JPG size (Which average 4,000 KB.) would fill 1 GB in nine seconds.  In 36 seconds your 4 GB FAT32 card will fill and then need to resume with a few dropped frames.  In other words, do not expect much from a screen capture off 4K over still.  Just ain't gonna work.