Yes  4 crew is standard but some sail with 3 and some with 5. The Ensign is not very sensitive to being overweight , so if you have a heavy crew it will hurt you in light air, but nowhere near as much as higher performance boats.  If wind is over 12 or 13  more weight helps you flatten the boat upwind and you will go faster.  In 18 4 big beefy folks if they hike are super fast. The total weight of the crew is not  much of an issue, What is an issue is what the crew does with their weight. 3 good hard hikers will beat 4 who sit on the benches.  Crew weight should be as forward as possible almost always, especially in light, downwind in light air a crew on the foredeck or down in the cabin is fast.  Crew that dynamically move their body weight with changes in wind strength are fast, they also need to dynamically keep adjusting sail settings with every puff, lull, course change etc. 

If you are light and in heavy air use all the techniques to de power the sails but keep them balanced so you get some power out of each sail vs dumping main and sail jib only. Jib car back, Jib 8 inches off the spreaders is a starting point  when you are way overpowered