Aussieman,
It's actually really easy, because the main MSFS files are pretty small compared to all of the rest. If you're okay with the base program being on the smaller drive, during setup, just create a folder like D:\FlightSim on your 4TB drive and choose that during the setup phase where it downloads the rest of (most of) the files. The small base executable will be on your C: drive but all the rest, and your Community folder, will be on D. That's how I have mine set up.
If you still have the Official and Community folders from your old install, you can even copy them in the D:\FlightSim (or whatever you call it) folder before you install the sim, then when you do the installation it will see the files already there and not re-download them.
If you Install MSFS with the Xbox app and you DO want to even put the base game on your second drive, before you install MSFS, go into the Xbox app, right-click your Gamertag in the top left corner, and choose Settings. Then under "General" you can change the drive and folder where games are installed by default, and also choose "Ask me for install options on every install." (If you set up a custom folder there, though, don't put the MSFS data files in that one -- it should only be for MS Store/Xbox App executable programs.)
Hope this helps!
Man, could MS make this any more complicated? Well, sure they could.
But it really makes me wish the Commodore Amiga hadn't died!