Information brokering: the commercial advantages of J4+, and somewhat less so J3, in a universe served primarily by J1 and J2 freighters, are not in moving cargo quickly - its in moving information.

Consider the J4 courier serving a pair of worlds linked by a J1 route, but not an xboat line (which don't serve all worlds anyway). The freighter on a least time route takes 4 times as long - on a standard commercial schedule of 1 week jump/1 week port, it's eight times as long. For J2 and J3, the time is cut in half. Or, a broker with access to a J4 courier (preferably one with passenger space) can speculate without having the cargo on hand. She simply buys the cargo at home, contracts a captain to carry it, flies ahead with the guarantee of delivery, uses that to secure a second cargo (purchased using financial data that's only a week old) and delivery, flies back with the second guarantee, then makes that sale. Time elapsed: about 4 weeks. Her first cargo is halfway there, while her second cargo (and the actual profits) are another 7 weeks away.

If she doesn't have a local gaurantor, she can still spend those 3-7 weeks (depending on least-time or standard commercial scheduling) lining up the best sale possible - and until the cargo arrives or the courier comes back, she has the most recent financial data on her origin world available, which means that she can sell just that to local brokers doing the same thing.

It gets even better if she is the courier. Financial data on a world is cheap. Financial data only a week old, and already analyzed, from 4 parsecs away, is not.

So what should be the value of information?