The assumed 3-4 enemy limit is something I've only experienced in very open areas. And they seem to compensate for this by very quick spawns of reinforcements, so the wave of enemies is pretty consistent.
In certain areas it is MUCH higher. I did the assassin's quest in Southpaw Steam & Power and there were times when 6-9 enemies were simultaneously on radar and attacking.
It all seems to depend on what area you are in. After another full night of playing in several areas I really don't see the enemy count as an issue.