Write Unique Values Under Certain Conditions?

What I am trying to do is create a list of unique values under certain conditions. I will try to explain it as best as I can.Imagine we have four columns:

Column A contains a product, so for example Apples, Bananas, Oranges...
Column B contains names, so for example Peter, Carl, Linda...
Column C contains stores, so for example Supermarket, Cornershop, ...
Column D contains a number, so for example 1, 2, 3...

Now let's say I want to see who bought apples in the supermarket, I would like to query kind of like this:

For all rows where column a = "Apples" and column c = "Supermarket", write me Column b value and expect something like

Peter
Linda

if only these two bought apples in the supermarket.

But I need to make sure its only unique people, so if Peter bought apples at the supermarket twice, I do not want to see

Peter
Linda
Peter

Now the next step would be to see what was the maximum amount of apples that peter has ever bought at the supermarket, so Im thinking of something like:

For all rows where column a = "Apples" and column b = "Peter" and column c = "Supermarket", find me the maximum value that is ever displayed in Column D.

Like I said, I'm totally new to VBA so I am not really sure how to get started with this, but what I do know is that doing this with if statements would take me forever, so I want to give vba a try.