The best upgrade for a Mychron 4 is the Datakey, enabling further analysis on a laptop. However getting to the connector on the back can be difficult in some applications, this is where the Data hub steps in.
The hub can be positioned in an easy to access place making getting the Data key in and out a breeze. It also allows the Mychron to run off the starter battery thus eliminating the use of 9 volt batteries. This has the side benefit of being able to check the starter battery charge on the display !
All this makes the Data hub the second best upgrade for your Mychron 4 / 5 system !