USCG "6 pack" licenses are primarily for commercial use ie., fishing, diving, & sailing charters for hire. The license is a bit costly to obtain because you have to obtain TSA security background checks and have to either take a classroom course and test from a USCG recognized commercial school or take the test at a USCG testing facility. You must also show evidence of a minimum number of days of experience on the water. The license has to be renewed periodically at more expense.
I believe what you are interested in is gaining the necessary experience and "certification" to charter boats at locations other than your home port. There are a number of American Sailing Association schools that conduct a broad range of certification classes. It appears that the trend at most schools is to combine ASA courses including "basic coastal cruising" (ASA103) and "bareboat cruising" (ASA104) into a one week long experience while you are living aboard the boat. Some of the topics covered include systems, engine operation, docking procedures, sail trim, navigation rules, basic chart reading & navigation, anchoring, provisioning, use of the galley, onboard maintenance, emergency procedures, weather interpretation, & dinghy/tender operation. As other posters mentioned, a lot of charter companies may not require these certification; however, you will definitely be more comfortable & confident in your capabilities aboard a larger boat if you plan to charter. I did the ASA 104 class about 10 years ago at Lanier Sailing in Pensacola Florida. The class was conducted on a Beneteau 393 (3 stateroom/berths & 2 heads). The instuctors and boat are first rate. I learned a lot and had a lot of fun. It was certainly worth the time & cost; I would highly recommend it. You will find that operating & docking a 40 foot boat is much different than yours. Having the certification & experience should keep you out of trouble and allow you to have more fun while chartering.