I corrected your post title as you referred to beta 5, but you had installed beta 4.
... when trying to border more than one table-cell (to make a border to all marked) only the first cell is bordered
Please try JCE Pro 2.9.99.3 Beta 5 - https://www.joomlacontenteditor.net/downloads/editor/pro/development/jce-pro-2-9-99-3-dev
Clear your browser cache after install.
Ryan Demmer
Lead Developer / CEO / CTO
Just because you're not paranoid doesn't mean everybody isn't out to get you.