Which is the most efficient Juicebox product to use to generate galleries from Lightroom 4 with the captions metadata?
In answer to this question, the Lightroom plugin would be most suitable (and is the only method of creating a gallery within Lightroom 4). You can select metadata such as the IPTC Caption or Title to be used as the Juicebox caption or title from the drop-down menus in the 'Image Info' section.
You would then have to upload your gallery folder to your web server and embed it in a WordPress post using one of the two methods documented here.
However, you could create a Juicebox gallery using a NextGEN gallery as a source of images from with the WordPress Dashboard by using WP-Juicebox (the dedicated Juicebox plugin for WordPress). Create a new post, click the Juicebox icon, select 'NextGEN Gallery' as the 'Image Source' in the settings window that pops up and type in the numerical NextGEN Gallery Id that you would like to use.