A pump track is like a 4x track but you normally wouldn't pedal that much and you would gain/keep your speed by pumping the backside of doubles/rollers, also they're normally only one or sometimes two lanes so you couldn't really have a mini fourcross race.
Were starting on one, 150 feet in length in an L shape, first person seen putting a pedal stroke in gets a spade round the head I'll get some pics up soon
The local skatepark here has some of the most perfect rollers around, you go into them with just enough speed to roll over the first, and you come off the second absolutely flying, a track with good rollers like those could be so much fun. I have never ridden a pump track, but BMX tracks are nearly as good!