Need a little more information. For example, is it just the pork itself, or will it be served as sandwiches? What kind, and how many sides? Any other meats? And will the plates be be prepared by you/staff and served, or is it just going to be a big buffet line?
Those items above will dictate exactly what you want to plan on, but for a rough start, you can generally plan on about 1/3 lb of finished product per person. So, 400 people, 1/3 lb each (rounded up to 5.5 oz), that means you are looking at around 138 pounds of finished pork. When you factor in about 50% yield, that's basically 275 pounds of raw pork butt.
So, from there, you'll need to adjust the quantity based on the rest of the menu, how the pork will be served, serving style, if you think they will be really big eaters, etc.