... from my memory a mate of mine had the original Boxer which was a C kite (circa 2006-ish) but they went to a SLE or bow style after that which means they had a biridle and only worked with 4 line bar. The C kite version, which I believe you have, "can" fly on 4 lines but can also be a bit wobbly in its flight characteristics. It especially becomes less stable when you load them up, they jellyfish. With the 5th line the leading edge is supported so it doesn't wobble about so becomes way more stable and feel better.
The other benifit if you stick with the 5 lines is relaunching becomes way easier. When used with a 4 line bar you need to swim towards the kite to flip it onto its back so you can relaunch it, where as with the 5th line, just pull towards you and it tips over allowing you to relaunch from there,,,, and trust me,,,, much easier than swimming towards it when your buggered!
Hope this helps!
cheers,
Robbie