A TF4 can be used with any decoder that is not transponder equipped to add transponding capability to the loco. It can also be used in any piece of rolling stock with track pick ups to add transponding. For example, you could use a TF4 to add transponding and lighting effects to a caboose if it is equipped with track pickups to provide power and signal to the TF4.
A TF4 should not be connected to an existing transponder equipped DCC decoder unless the DCC decoder's transponder is disabled. This avoids having 2 transponders responding to the same address in the system.
For example: If you are using a TF4 to add more functions to an existing Digitrax or other transponder equipped decoder, you must disable transponding in the mobile decoder to avoid having two transponders in a single piece of rolling stock.