Create unique items in valiadtion list

Hello everyone
I have a validation list of three items (A,B,C)
these values are in column A ..
In E4 I have validation list of the three values (A,B,C)
The required in F4 to create validation list of unique items for specific item
As an example if I select A in cell E4 , i expect to have a list of these values(20,5,9,19) only
Hope it is clear