Sadly, that's actuall the entry spark has for "walk away the pounds" (the aerobics low impact one). But it gives MORE burn than walking 4mph.
Frankly, I'd stick with logging it with the walking. It's a lower burn and much more realistic.
CHEETARAH is right that walking in place just is not the same as walking in space, so you should not use Spark's own listing for the dvd, which gives a higher burn than even plain walking.
If accuracy is important, you need to buy and wear a heart rate monitor.