I visit my relative in Karlsruhe regularly, had just spent the last 3 days there. 3-4 hours is enough to see the interesting part of Karlsruhe on foot. I'd recommend the following: store your luggage in lockers at train station (Hbf), walk out of station's main entrance/exit to the Tourist Information office directly across the street, pick up city map from friendly English-speaking staff, ask them to mark the lovely walking path thru the green parks to the Schloss, you'll get to the grounds of Schloss Karlsruhe in about 15-20 min by foot. The grounds of the Schloss is quite lovely in spring and worth a stroll, you could pick up food on Kaiserstrasse and have a picnic there (many benches, especially around the lake). I find Schloss Karlsruhe (4 Euro) visit-worthy b/c it is about the city's history, high quality well-arranged display with lots of historical items. It takes under an hour to visit. The main shopping street is Kaiserstrasse (5 min walk from Schloss Karlsruhe), particularly busy b/w the tram stops of Marktplatz and Herrenstrasse. My favorite place for a quick sit-down meal is the cafeteria of the big department store Galeria Karstadt (right on Kaiserstrasse in the middle of these 2 tram stops) --lots of hot entree/hot&cold salad/soup/dessert/juice choices, spacious dining room, very clean WC. There is a good supermarket in the basement level of this department store. If you like German sausages and porky items, the butcher/sausage shop Schlemmermeyer (right at the Herrenstrasse tram stop, at Kaiserstrasse 100) has good quality hot meat item/sausages/typical regional German salads (sold by weight). At the Herrenstrasse tram stop, there are several good bakeries and places to pick up beverages (DM/Müller/Rossmann drug stores). S1/S11/3 tram goes from Herrenstrasse to Hbf in 10 min (2.6 Euro single ride, buy ticket at tram stop or on board).