:-) Have a look here. -
https://www.brewersfriend.com/refractometer-calculator/ it's a bit confusing, and to get it totally accurate you need to do a bunch of small tests to calibrate your refractometer.
Alcohol affects the reading of a refractometer making it read artificially high. I've not used one myself but done a bit of research as I want one to allow for quicker readings during brew day and not needing 50 - 100 mls per sample during fermenatation, which is especially annoying if doing a 1 gallon batch of mead.
Assuming no correction factor I plugged your figures of 1.039 (9.75 brix) and 1.017 (4.25 brix) into the 2nd calculator and it comes out as an adjusted FG of 1.008 and 4.13%, which is 79.5% apparent attenuation and pretty good.
Hope this helps.