If your old C drive goes back a few years they might be there from when V4 needed to be in the main runtime which is not required now,  Does V4 exist is other runtimes?  If V4 does it should be safe to delete.  I think if I was in this position I would delete in stages so that the files went into the recycle bin and then try and load V4 in Poser.  If it does not work you can always restore from the recycle bin.  OR  If you have the space elsewhere you could do a move to a temporary folder and then move back if required.  Ideally the SSD should limited to the OS and programs and any of the temporary folders generated by Windows or your browser and the like moved to a conventional drive. This not only saves space but it also reduces number of writes to the SSD and thus extends it's life.

 

 

I use Poser 13 on Windows 11 - For Scene set up I use a Geekcom A5 -  Ryzen 9 5900HX, with 64 gig ram and 3 TB  storage, mini PC with final rendering done on normal sized desktop using an AMD Ryzen Threadipper 1950X CPU, Corsair Hydro H100i CPU cooler, 3XS EVGA GTX 1080i SC with 11g Ram, 4 X 16gig Corsair DDR4 Ram and a Corsair RM 100 PSU .   The desktop is in a remote location with rendering done via Queue Manager which gives me a clearer desktop and quieter computer room.