You can visit the Trek archive to help get your bearings with respect to when the 400 was made. If you find one used, check the color on this list and you'll find the year it was made.
Trek Bike Models by Year and Color
There are tons of used steel road bikes available on craigslist. The nice thing about Trek is the catalog archive(scroll up on the site above..lots of info available). You can track nearly any older, steel bike back to year and components from an ad by knowing the model and color. Trek also had big sales numbers..so you'll see them used.
If your dad tends to like sitting upright more than the road bike (drop bars) posture, look into the Trek 7.x FX series. (7.1, 7.2...etc). The 7.5 is a nice, fast, comfortable bike with decent tires on it(dump the hardcase tires they come with..replace with Panaracer Gravelking slicks 700x35).