If you think the a fail safe is good enough you are wrong. If your throttle servo mess up or the linkage comes off what is a fail safe going to to NOTHING. But with kill switch you can just shut down the engine. Go to Killer RC and look in the FAQ section for more. Just think we are talking about vehicles that weight 20 to 35 pounds not little 1/8 7 or 10 pound vehicles.
I know of a guy that ran his 5B off in to the ocean. He had a fail safe but (no kill switch) did not do him a BIT of good. He runs now. Losing your 5b is bad but what happens if you have a problem and it hits someone or there property? LAW SUITE! Now what is cheaper? Mess up or loosing your large scale RC, law suite or a kill switch.
I will not run any of my large scales with out one.
I recommend going with the #4 as the LED light makes is A LOT easier to tell if you kill switch is on or off. I ran a older kill switch with out a light and is a pain in the butt. Get the kit with LED. Kent at Killer RC is a great person and give great service both during and after the sale.