Numbers don't always tell the whole story. Seat shapes can make small spaces seem big, and big spaces seem small. The only real way to tell is to actually compare them in person.
That being said, the back seat in an A4 is for children only. Especially with a tall driver up front. Your knees will be slammed into the back of the drivers seat. It's also a pretty narrow bench so you will have a tight fit getting three people back there. The Accord will be dramatically more roomy.
There is a considerable amount of room for the driver because of tilt & telescoping steering wheel. VW's/Audi's traditionally have some of the furthest sliding seats out there. My Passat seat would slide so far back that I could barely touch the pedals (I'm 6'). Most Japanese cars lack that freedom (Honda, Nissan, and especially Toyota have this problem).