IEEE 1149.4 was released in 2000, with the objective of providing a chip-based solution to perform analog measurements on printed circuit boards. The dot4 specification adds 2 pins to the TAP, an analog drive pin called AT1 and an analog sense pin called AT2. Resources inside the dot4-compliant IC allow AT1 and AT2 to be connected to analog test buses and from there to analog pins on the IC to be driven or sensed. Resistor values, capacitor values and voltage levels can be measured as long as there are dot 4 compliant devices on the board.
A good way to learn more about the capabilities of analog boundary-scan, check out the JTAG Technologies Evaluation kit, complete with a demo board and analog instrumentation.