Here is the list of known differences between A2J Author version 4.0 and A2J Author version 6.0:
Phone number and SSN field no longer force a format on the user's input
Authors cannot turn off navigation
Variable types must match in order to add the variables together
True/False variables are Boolean now, so no quotes around "true" or "false" in logic statements
Nested repeat loops require an additional designation of the outer loop counting variable within the inner loop questions
Hyperlinks and Pop ups are now blue, not red
Arrows are used in the navigation bar, not "Next/Back"
Authors need to reset hyperlinks in order to force them to open in a new window. Otherwise, they'll open in the same window as the viewer.
[Exit-User Doesn't Qualify] doesn't close the viewer window if the author has not included a re-directing URL