400 dpi is unnecessary for the most part
As a professional graphic designer and large format digital printer I can tell you that 400 dpi is completely unnecessary as far as maps are concerned. While many large format printers print at 720 dpi and greater, high resolution is a factor only regarding clean lines and very small text.
For photographic work, including maps, 200 dpi is ideal for printing. I have printed large format CC3 files, that included hundreds of very tiny labels for villages and such, in the size of 8 point letters. For really small text 300 dpi is preferred. But unless you plan to use very small labels, you don't even require 300 dpi for your map.
I'd say stick with 200 dpi in your map design, you'll have much more freedom in your cartography if you use a doable resolution. Throw the 400 dpi out the window, it offers you little benefit for the expense of working with an unnecessarily huge file.
Believe me, I am a professional fantasy map printer, I know what I'm talking about. I'm the exclusive printer/distributor for a number of small RPG publishers. Go for 200 ppi...