If you're meeting at Union Station, you'll likely be hungry. So, I recommend that you eat at Lou Mitchell's, as it is a classic Chicago spot. You can actually rub elbows with the locals. See http://www.loumitchellsrestaurant.com/ . It is a few blocks from Union Station though at 565 W Jackson Blvd. Another good place is Arturo Express (http://www.arturoexpress.com/aboutus.html at 122 S. Canal St. - basically Canal and Adams St.). They made the best tuna sandwich that I'd ever eaten. Also, you'll really get the Chicago accents and vibe there. They serve fast, quality food at reasonable prices.
After that's out of the way, you all can walk across a scenic bridge over the Chicago River, then take the tour up Willis Tower (http://www.willistower.com/ ) on Wacker Drive. The tower has a new viewing area where you can step out and feel like you're walking on air.
Then, you can walk or take a bus east to State Street. Do some shopping (window shopping, if you prefer). Macy's - formerly Marshall Fields - is there on Washington & State.
You can cross Michigan Ave. at Washington and walk left to check out the fairly-new Millennium Park. It's fabulous. See http://www.millenniumpark.org/ for details. It stretches between Randolph St. and Monroe Street on Michigan. Otherwise, you can cross Michigan Ave. and walk just right to explore the excellent Art Institute of Chicago (http://www.artic.edu/aic/ - right there on Washington & Michigan). They just built a new modern-art wing last summer, which is worth checking out. Unfortunately, the free hours don't begin until 5 p.m. that day, but if your boyfriend has a Chicago Library Card (which he can get if he has a suburban one) then he may be able to check out a free Museum pass from a Chicago Library before you get there.
The above will probably kill more than 4 hours of time. ;-) I recommend stopping your activity about a half hour or hour before 5 p.m. (depending on where you're catching the train & whether you take a taxi back or not).