The term 'beta' has slowly drifted over the years... I'd consider them to be release candidates at the least, and probably stable with what's currently considered stable being more like long term support.
But apparently there's a few bugs with Mantle support in BF4, not the AMD drivers. Something about over-allocating the VRAM on certain cards, so they might have disabled it in some.
Also, you need to have COMPLETELY UNINSTALLED the previous drivers first. That means booting into safe mode and using something like Driver Sweeper, or ideally a full reinstall.