The last three years my wife and I have been placing the lever on the '4' position instead of the 'D' position. I am not sure if this was how the salesman explained to us as we bought the van new.
Well, there's the first mistake: listening to the salesman!

Sales staff know how to sell... and in some cases nothing else.
There are a lot of better sources of information:
- the owner's manual, which has a clear and descriptive section on use of the transmission shifter
- common sense (why would there be a "D" if it is never used? "D" means "drive")
- even forums like this
No real harm done. Yes, the mileage was pretty good in "4", but I'll bet it would be even better in "D".
So, does anyone need a description of what the shifter positions mean?