Inkscape 1.4 has installed but won't open,

can't roll back, can't access any inkscape file :(

I had the same problum.  You may have to go back to the Install screen (as if you are going to install Inkscape for the first time and at the prompt whether to install or  Repair chick Repair. Follow the next instructions.  Then It may seem to take a long time (it did in my case) but eventually the repair works and you can accept the version, as usual.  Then you should be good to go in ver. 1.4.

Or  uninstall what you have  of Ver 1.4 (and you did uninstall any other older versions of Inkscape prior to Ver.1.4 Install didn't you?) then reinstall 1.4 again.
Hope this helps

Of course I didn't uninstall any older versions of Inkscape ( I just assumed the install process would do that automatically)

However the repair option solved the problem

Many thanks