All the William IV sixpences I have seen have had an inverted die axis and there is no mention of this in either ESC or Davies, so this is clearly irregular. First of all, is it genuine? There are quite a number of Geo.III forgeries about, but I haven't seen any William IV. Post a decent picture to establish whether it is genuine and we can take it from there.  here is a scan of the coin they arnt the best scans i could get but i had to downsize them which has made is low quality