You did mention you are using a 4k screen so we will start there. Unless you have been judicious with your setup, it is possible that you are running 5x WoW @ 4K resolution, and in this case, then your current graphics card may not be coping with this. This should be simple enough to determine by doing a bit of monitoring, and seeing if the GPU Engine/Memory is maxing out (e.g. pipping above 95%). A method on doing this is covered in the
tweak your framerate page.
If you find that the graphics card is topping out, or you don't want to do the monitoring, your Window Layout in your ISBoxer profile will show the resolution of the Active Region (usually Region 1 in the standard setups). If this is set to 4K (or close to it), then that is probably the issue.
Regardless of which identification method you use, you can use the Window Layout Wizard to create a new Window Layout, and try setting the 3D Render Size option in the lower right of the wizard options. You can set this to something more manageable for running 5x WoW, e.g. 2560x1440, or even 1920x1080.
If that doesn't solve it, the most common reason for freezes is some other software which also happens to be running at the same time on your computer, and that other software doesn't play nice with Inner Space. So get a
compatibility diagnostic, and we can have a look, and go from there.