Yes, Lr v4 will work with Vista with Service Pack 2, but Adobe doesn't sell it now, so you will have to look for it on sites such as eBay and Amazon. On these sites, you will mostly find full versions but you may get lucky and find a French upgrade version.
You can download the 30 day trial version and install it. But when it comes time to purchase there will not be any way for you to purchase Lightroom 4. Adobe only sells the current version. That download is provided primarily to enable users of the older version to download if they have a computer crash or purchase a new computer.