There have been known issues with 4-cell racing and the PT's. Depending on what class you're running, stock, spec or mod. you may want to consider running a receiver pack to insure consistant voltage to the PT.
I don't think the 5volt bec would be an issue since a 4cell batt. only has 4.8 volts available. I think its the amps of the bec that is more important. I have 4 GTS controllers, I don't think you can beat them for the price. I've never had any problems with them but I don't run a PT.
If you solder in your batterys or use a connector ,,, just cut the end of your PT and solder dirrect to the speedcontroller wires .
That way you'll have plenty of current and the least amount of voltage loss to the PT.
Your bypassing the ESc and reciever but with only 4 cells your perfectly safe.
Their rated for 4-8 cells ,,,, but as the voltage drops at 4 cellls your probably borderline.
If you can afford the weight gain , buy a 6 volt reciever pack from Pro-Match and its about the size of a quarter and .6 oz.