4 or 5 lines, they both work. The SS single frontline safety works like a charm, most of the time, as does the 5 line safety from North. Just don't fumble with the stopper balls and you will be fine.
However, the argument that 5th line will cut through a kite in waves is not a valid point, as any kite will be cut to pieces by the lines if you are unlucky. Had that exact scenario with my first RPM being eaten by the waves.
Since I kite I had a torn frontline twice. Once on the RPM and once on the Rebel. On the Rebel: No problem, pulled the safety, that was it. On the RPM I got lucky as hell, because it was not the safety line being ripped. Otherwise the kite would have been gone over the big blue ocean
And the "oh-shit-handels" don't qualify as solution for a torn front line safety, simply because try to use them in high winds, you won't try it again...
There is only one reason why the 5th line might be annoying: You have to attach ONE MORE LINE. I hate that so much my blood starts to boil and I don't have to do a warm up afterwards