To what GNVE already said, there's also two things to consider:
Firstly, the GPU is used when needed, and as much as needed. If you do a test render at low setting, with just a figure and some clothes, the needs will not be the same than if you rendered a really complex scene at high setting and with many reflective and transparent materials. When I'm testing with Daz, I rarely hit 100% cuda use, and they are rarely used for more than 5 seconds. For a 4090 it would be around 20% cusa use for one second.
And it's where come the second point, Windows percent is the GPU global use, and the said GPU isn't limited to its cudas. There's also, among other things, a part dedicated to video encoding/decoding, that is totally useless when rendering. What mean that even if you were rendering something that put the cuda use at 100% for a whole minute, the GPU use wouldn't be at 100%.
If you want to have a better view of the use without using GPU-Z, just configure correctly the task manager. The four graph windows are configurable through the arrow on their top left. Put one to show you the cuda use, and you'll see if Daz Studio effectively use or not the full possibilities of your 4090.