Over the course of 10-20 years, I think 4% may be somewhat on the low side. I'd calculate more like 5-6%.
The increases have been steeper in the last year or two than the preceding 3 or 4 years, but averaged over about 10 years it's probably close to what you're coming up with.
Historic figures are a little difficult with WDW resorts, because they're adding them so quickly and because only 3 or 4 resorts go back to the beginning.