Every city in the usa has a mall or four.
Here,
A mall is indicating a shopping mall
But what does four mean? Is it indicating 4 malls or something else?

Question

Every city in the usa has a mall or four.
 Here,
A mall is indicating  a shopping mall 
But what does four mean?  Is it indicating 4 malls or something else?

Answer

I agree with Eric.   To add a bit more . . .   "A something or TWO,"  is a common expression.  It can be applied to almost anything.  So, the joke is that he says FOUR instead of TWO, meaning there are a lot of malls.

Answer

Hey Alam! Great question. Here the speaker/writer is making a joke, saying that there are many malls in US cities. It's basically saying, "Let's be honest, there is usually more than one mall in an American city."