[wesk05]

Can current HDR displays even get to 4000 Nits ?
What happens if they cannot for "those" specific titles ? 

There are no consumer displays available that can output 4000 nits. This is where the mastering display luminance data in the HDR InfoFrame comes into play. It tells the display to adjust its tonemapping function. ITU Report BT.2390 has elaborated on how a display can normalize the PQ values based on its own maximum output and the mastering display peak output. Display manufacturers could be using a similar function.