Disable external module to preserve battery life

[jonnyfishman]

Second question for the evening. I have tried several versions of the circuit below to disable an external RFID module when it is not needed (it is triggered by a button press).

The left circuit it the standard one for an RFID module but I have tried to disable it using a BC548 NPN transistor and D4 pin set to LOW when not in use (right circuit), but I can't get it to stop being powered. The LED responds as expected and turns on an off depending on D4's state but the RFID module always seems to be powered. Is this likely to be because of its other pins MISO etc, or because D4 isn't setup properly.

N.b. I have tried without the transistor and the module remains powered regardless of D4's state.

Thanks in advance,
Jon