The rule is in place to prevent pvp-whores I guess. I know its a clunky rule, with different rules for traders and combat vessels... But it works. Maybe traders should be forced to find a new route until the 4 hours is up/pirate leaves the system, but that's another discussion entirely.
Captainpoo: If one person leaves the fight for any reason, with cruise engines or in an escape pod, it counts as a "death".
Also, if a battle is called a draw, I would not reengage the other guy for 4 hours, as I just fought him. But I'd remain in the system, as the fight has no real winner...