Populating Dropdowns in Excel dynamically

Hi,
I've a requirement of populating different tourist destinations of a particular state in 4 different dropdowns. The input to all 4 drop downs are same say A, B, C, D (all dummy city names). Issue is that if user select one among A B C or D from first drop down ,then second drop down should not contain the city user has selected in first drop down.
Ex. In Drop down 1 user will have all cities A B C and D. Suppose if user selects city B in Drop down 1 then Drop down 2 should show only A C and D. Similarly if user selects City C in Drop down 2 ,then Drop down 3 should have only A and D like wise..
I'm stuck with this issue, any help greatly appreciated.
Thanks in Advance.
Anjanayan