Small screen sized 4K monitors (laptops) can suffer this if your default Display Settings, Scale are lower than the recommended %.
Plus Avast isn't particularly good at handling hi-res monitors (very poor would be a better description), there has been one other topic at least that I have seen about this UI windows too small.
Even on my 15.6" laptop on 1080p I have to make compromises on the default settings (recommended percentage 125%), mine set to 100% or I have the reverse of your problem (UI too big).
The fact that Avast have been totally unresponsive about making UI screens user resizeable and has fallen on deaf ears.