AFAIK DP 1.3 exists on paper only (and in GPU theoretical "driver support"), and maybe in some prototype lab demos. Nobody managed to mass-produce stable working DP 1.3 controllers and cables yet, not to say 1.4
I don't know much details, but suspect greatly raised bandwidth was made it too susceptible to interference (which was somewhat in issue even in DP 1.2).
Bandwidth required to drive 4K@120Hz is quite massive - to give you idea, it would occupy majority of Thunderbolt 3 connection (and Thunderbolt 2 is not enough). Unlike DP 1.3, mainstream Thunderbolt 3 hardware
does exist already and
can be used to produce 4K@120Hz display - but monitor manufacturers are probably reluctant because adoption of TB3 ports on PC side is still low...
Another real possibility is to use DP aggregation to use
two DP 1.2 connections to drive
single monitor. This was actually done by Dell for UP2715K to drive 5K with two DP 1.2 cables & ports. But nobody else did it - again, probably because solution is too non-mainstream and fragile
