D5 is serious. It's for the large motor coaches, city busses, sailboats in the 40-55 foot range.
The D2 will keep a Sprinter type van plenty warm. Lots of D2s in Sprinters prove that. But it will work harder than a D4. Where the D2 might spend most of its time on Medium and High, the D4 in the same vehicle will spend most of the time at Low and Medium.
And as a result, over its lifetime the D4 consumes less fuel than the D2. It's not a big hairy whole dramatic lot, but it makes the cost difference between the two about a wash over 5 years.