Hm? I tried looking for it but everybody posting about it is already familiar with that model. What's the difference to the M200? Size? Nib? Better material?
First of all, all the 2x/4x is the same size (within less than a milimeter variance).
400/N/NN vs M400: 400/N/NN come from the 50's 60's. Barrels are (IMHO) prettier and higher quality as the stripes are there in the raw material composition. The main difference is the nibs. The older ones are usually more flexible with more "italic" cuts which are a joy to use (and they use ebonite feeds). M400, while technically starting by the end of 80's, can be considered "modern", since most units you'll find will probably have less than 10 years (and you can buy them new too). Nibs are rigid and nib points usually quite "blobby" (and they use a plastic feed); very easy to use by everybody but lacking "personality". Their barrels are made of transparent plastic with a convering binder that "fakes" the stripes.
M400 vs M2x: M400 have gold nibs while M2x use steel nibs (with some exceptions like M250, which was basically an M200 body with an M400 gold nib). M2x plastic and finishes feel somehow "cheaper" than their Souverän counterparts (Souverän is the common name for all the M4/6/8/1000 series). Don't be fooled for the "steel nibs" part: some people will tell you that Pelikan's steel nibs are better than their M400 gold nibs (me included). Of course, being steel, they lack the "bling factor" of the gold ones, but their perfomance is better than their modern M400 gold conterparts, and (again IMHO) even are more beautifully shaped (longer, more "fluid" tines).
Then you have the M215 series which is an M2xx (cheaper model, steel nib), only with a bit heavier barrel, which I happen to prefer because of that.
Of course there's a lot more that can be said, but those I'd say are the most obvious differences.
A big advantage of all these models is that their nib units can be exchanged along all the range. To me, the best combination of all them would be a 2005 model M215 blue barrel, with a gold nib from a before-1954 400 model, with a close sencond being a first series 400 (1951-54) on tortoise-shell finish.