I am also having the same issue.
I performed a full reinstall last night, including deleting all the AppData & ProgramData content for P3D.
The 777 & 737 work fine on 4.2, however when loading the 747 from the scenario screen, it crashes. I can however load in the 747 after loading in another non-PMDG aircraft first. I thought it might have been an FSUIPC issue, but when I deleted FSUIPC the issue still remained. This is all that loads in before it crashes: (shader issue maybe?)
I currently have the following addons installed for reference to others who are having the same issue:
Orbx FTX Global, Europe OLC, FTX Europe | UK2000 VFR Airfields Volume 1 (EGHH Pictured Above) | ASP4 v6616 Open Beta
I realise that what I'm saying is almost identical to what has been said before, but I just thought I'd provide more evidence of my experience to help the team gauge an idea of what's going on.