Afternoon!
What size of hammer are you using? Stuff like that needs a big hammer and not some B&Q DIY toffee size thing!
All posts above correct, i wouldn't bother with the grief of trying to fit 2WD brakes to a 4x4 hub, it's not worth the hassle and if it was 'easy' it would a be common uprgade.
There shouldn't be any kind of retaining screw on the disc to hub, only thing I can suggest is there may be a spigot ring adapter or something thats also welded on with rust that's stopping the disc coming off, but even then it would come off eventually with a few hits.