What is the difference between a PT3 and a PT4 power lead?
The two leads are NOT interchangeable.
 One is the "complement" of the other.
 The two will connect together as "plug &amp; socket"

PT3 is referred to as a female connector.
 PT4 is referred to as a male connector.

The PT3 has a hollow centre and it connects to a PT4 which has a 2.1mm "spike" in the centre.

In the photos below note how the PT3 and PT4 can be fitted together. 

This photo shows a PT3 connector and cable



This photo shows a PT4 connector and cable.





This web page  says:


PT-4 power leads are typically used to connect the 18 guage pair of power cable to RG-59 siamese cable to a security cameras DC power plug input. 

PT-4 power leads can also be used with pro box style security cameras that do not typically have a DC power plug attached to them.  Box security cameras typically have screw terminals for a positive and negative wire connection.  If you are using plug and play cables, you can attach a PT-4 to the cameras power terminals for a clean connection to your plug and play cables.  

PT-4s provide 1 foot of cable and a male connector for CCTV Camera power.
The diameter of the connector is 2.1mm which is the standard size used for plug and play cables.


This web page says  says similarly, except 


... PT-3s provide 6 inches of cable and a female connector for CCTV Camera power.
The diameter of the connector is 2.1mm which is the standard size used for plug and play cables.