There is no beta version of the jQuery Mobile Themeroller for 1.4.
Jquery Mobile's Blog States:
We are also still working on updating the ThemeRoller for 1.4, which takes more time than usual because of all the changes we made in the theme CSS.
The latest CSS Version rc1 style sheet can be linked to here.
<link rel="stylesheet" href="http://code.jquery.com/mobile/1.4.0-rc.1/jquery.mobile-1.4.0-rc.1.min.css">
This stylesheet consist of only:
data-theme="a" (Light - White Theme)
AND
data-theme="b" (Dark - Black Theme)
Judging by the timing of releases I suspect the Themeroller 1.4 will be released around January.