4GB free disc space ?? this is a problem. I assume you have default pagefile configured and these is located at windows drive.
Independened of the pagefile, you should ensure that you have at least >30GB free disc space ( and this is realy a minimum, better is much more like 100GB free ) … You can start windows cleanup , etc. … Windows itself, and also the applications need space to write e.g. some temp files and you run into strange behavior / issues in case there is not enought free disc space.
May be these also prevents the download… or maybe its the pagefile size.
With 16GIG RAM, you need possible much more than 5GIG pagefile ( good topic about ). It depends bit on your game settings/resolution. In case you have two discs, you can configure on each disc a seperate pagefile. Best you should ensure that you can get 16GB additional pagefile. ( 16GB RAM + 16GB pagefile )
Example Pagefile Setting with assumption limited space on C: and existing D: with enough free disc space
C: Minimum Value : 1024 Maximum Value 4096
D: Minimum value : 8192 : Maximum Value 20000