My old post on the history of Four Thieves Vinegar has had a few tiny updates and corrections made.
Four Thieves Vinegar is used in conjure work for a variety of purposes. Some people use it in spells meant to harm others or to "sour" another's fortune. Zora Neale Hurston described it being sprinkled around as part of the ceremony for a harmful magic spell. Others sprinkle it for protection and cleansing in their own homes, or add it to their baths.
The traditional use of Four Thieves Vinegar is more in line with the protective usage, as it was originally employed to prevent the spread of disease by being sprayed in contaminated areas, rubbed on the body or, sometimes, taken internally as medicine. Be wary about using some of the modern commercial preparations this way -- I know at least one spiritual supply company just sells vinegar with blue dye added as Alleged Four Thieves Vinegar; I was horrified to learn of one girl who'd been told to drink Four Thieves Vinegar as a home remedy for acne and wound up drinking the fakey blue-dye stuff without realizing that modern "spiritual" products are not the same thing as actual herbal medicine.
More information about using Four Thieves Vinegar can be found in the original posting here: http://spellcasters-source.blogspot.com/2010/06/four-thieves-vinegar.html