What are the selection parameters for an NPN BJT?
The R4 is there to limit the base current going into the NPN transistor. Once above the Vbe threshold the base looks pretty much like a forward biased diode to GND and will accept current to higher than is safe for the transistor unless the current flow is limited. 

For this application you could use almost any small signal switching transistor. Some popular, and inexpensive, types that would be entirely suitable would be the venerable 2N2222 or the 2N3094.