I am also using 4.0 beta 1 dev
Did you tried to copy site template or administrator template ?
I can't do it, I have an error :
JInstaller: :Install: File does not exist /home/bruno28/sites/joomla4beta1/tmp/template_copy_5e355972ce9ee/language
Template Install: Could not copy files from the files source.
Error installing template
Unable to install new template from temporary folder.
My temporary folder has 755 read/write property