There is no need to re-install content for DS4, you can simply list the DS3 Content folder as a DAZ Studio and Poser Formats folder inside DS4P.
In DS4P in the Content Library tab, right click on DAZ Studio Fomrats, and then select ‘Add a Base Directory’. Do the same thing with Poser Formats, and use Add a Runtime Directory. In both cases, you should give the path to the Content folder, and not any folder below it, DS4P will pick up the Runtime folder from there.
See if that helps.