Using a pressure cooker (6 qt) to can?
The USDA identifies a pressure canner as having a removable wire canning rack to lift and separate jars, AND a minimum capacity of at least four (4) quart-size jars... so that eliminates pressure cookers.
A pressure cooker, with its smaller volume capacity, is not recommended for use as a canner because it has a shorter heat-up and cool-down period. This shorter processing time raises the question of food safety.
Canning isn't the slap-dash affair it once was. Today's guidelines are very precise and science based. So don't cut corners when it comes to food safety, its not worth the risk of playing Russian Roulette with your family's health.
This should help; Download the U.S. Department of Agriculture's Complete Guide to Home Canning, its free, and an excellent source of tested recipes. Also, the National Center for Home Food Preservation, a reliable source for current research-based canning directions.
Don't know what your definition of small batches is, but you can get a smaller-sized pressure canner. You don't have to do 16 pints at a time, but consider that most low acid foods need 75 to 90 minutes processing time. I personally wouldn't want to keep my stove going that long for just four jars of something. I agree with Miss Vicki; get a real canner unless your Presto specifically gives instructions on how to use it safely as a pressure canner. I believe Kuhn-Rikon has pressure cookers in a variety of sizes that work as pressure canners as well, but they are not cheap.