Finally, someone is making a better 4mm bullet connector:
http://www.amainhobbies.com/product_...-Connector-Set
I don't know how long these have been around - but they are new to me.
The problem with just about every 4mm bullet connector is that all the current flows through little spring loaded fingers with relatively little contact area. They get loose over time and the resistance goes through the roof. It you let it get bad enough - the heat buildup at the connector will melt the female bullet tube right off your expensive lipo pack. I've seen this happen way to many times.
I applaud Castle Creations for designing a better 4mm connector - more in line with the super high current 6.5mm and 8.5mm connectors. They are a little harder to push in and remove, but that's the price for a better connection.