Ok, your original post said the trailer "should" weight under 6000 lbs...you didn't say how much under....could it be more? Also, you didn't say "where" you're going to pull this trailer nor did you mention for what distance and terrain. Wind can play an important factor into this equation also.
So, say 6000 lbs, then the 4200 dry weight is irrelevant and doesn't come into the picture. A 6000 lb trailer will have at least a 600 lb tongue weight and could have up to 900 lbs of tongue weight. All depends on how you load the trailer and what you put in it. For safety sake, I always err on the max side. Will your truck pull this? Yep, it sure will...but it will probably be an adventure. Keep the rpms up and watch both the engine and trans temperatures.
As I previously mentioned, I pulled a 5600 lbs trailer with a 750 lb tongue weight. My supercharged 00 4.7L V8 engine pulled the trailer very well, but I "always" knew it was back there and the slightest wind of passing trucks or nature would push me around quite a bit.....even with my hensley arrow hitch. I eventually wore out my 3rd member (rear end) bearings. I was never real happy pulling that trailer.
So, to answer your question, I would go with an 800 lb WD unit, or 1000 lb unit...that should be more than adequate. Be sure to know how to adjust the bars so that your truck and trailer is setting level.
Good luck
Ron