Good question – I pinged some folks internally about this as I am not seeing UE4/Engine/Extras/Redist/en-us/UE4PrereqSetup.msi/exe as part of the launcher install. I haven’t paid attention to this aspect in a while so the folks that deal with this will need to comment and correct me 
The TLDR on intent is that you shouldn’t have to worry about the prerequisites and we should have an easy way for you to satisfy them on user machines.