Hi all, I need some help with dealing with duplicates in my data.

I have something like this:
var1 var2 var3 var4
a x Red 1
a x Green 2
b y Red 3
b y Green 4

This is a simplified version of my data, most of them only have 1 unique var1 and var2, which I intend to use for merging data. But from some variables, there are different types of Var3, which separates them.
What I want to achieve is to add up var4 if var1 and var2 are the same, and drop every var3 != Red.

So the effect I want is:
var1 var2 var3 var4
a x Red 3
b y Red 7

I got a feeling that I should use for loop but not sure where to start.

Can anyone help please?
Thank you sooooo much!!!