Hi
no plan now for a VST 4, VST3 will be improved by adding some features.
For more info about MIDI 2 and VST3 you can check this presentation:
ADC 2020: Support of MIDI2 and MIDI-CI in VST3 instruments, Arne Scheffler and Janne Roeper
=> Arne Scheffler and Janne Roeper - Support of MIDI2 and MIDI-CI in VST3 instruments - YouTube
Cheers