What is the difference between the 4339A and 4339B?

The 4339B is the enhancement model of the 4339A. The primary enhancements are as follows:
(1) Backlit LCD
(2) Selectable number of display digits (3, 4 and 5 digits)
(3) Time count-down display mode in measurement sequence function
(4) Test current monitor display function
Front panel keys, handler interface and GPIB commands of the 4339B are identical with those of the 4339A.