Not sure why I didn't comment on this earlier. Digital zoom should always be avoided at all costs. Digital zoom just crops an area of the picture then expands it to the size of the full frame to give the appearance of being zoomed in. In reality what you're doing is taking a small part of the picture and expanding it. To give an example, if you Digitally Zoom in to an area that's equivalent in dimensions to 1080P, then you have a picture with 1080P dimensions. When you digitally expand that out to a 4K pictures dimensions, then you halve the pixel density (reduce it by 2 as a 4K picture covers an area 2 times as large as a 1080P picture). But the the picture density in the 4K picture was 4 times the density, so you're ok yes? No. The picture also covered 2 times the area. So the pixel density was double. So, you're left with the equivalent of around a 1440P picture expanded to a 4K size. Hence the drop off in sharpness. The further you zoom the worse the picture quality gets. eg. Get down to the pixel density of 1080P, then you're expanding a 1080P picture to be 4K in size and qaurtering the pixel density in the process. Anyone who's tried this knows the drop off in sharpness in readability and increase in noise. It's this doubling of density that enables the retrieval of more fine detail in native 4K. However, the more you zoom in, the the more you reduce the density back down to 1080p or beyond....
If you need a zoom on a camera, always buy one with an optical zoom. With an optical zoom, the pixel density is maintained and should you need to retrieve detail that's small in the final picture, then you have the head room to expand it slightly to try to retrieve that detail after the fact, albeit with the same effects as zooming digitally. However, record a digitally zoomed picture, and you've already used that headroom in creating the original picture so after the fact, further cropping becomes difficult to impossible.
Please feel free to correct me if I've made an error as this is tricky to explain.