I would take "next exit" to always mean the first exit you come to after this point - in other words, junction 5 in your example.
The only exception is if someone made the distinction really clear, by saying something like "not this exit but next exit". But that would only make sense if you were already passing, or very close to passing, the current exit.
It's invariably used in fairly lame "witty trading name/slogan" contexts, which as others have pointed out, all derive from the original "Toys R Us". The noun that precedes it is always something "we" (not "us") specialise in supplying.
I can easily imagine someone in a group of enthusiastic drinking buddies saying, for example "Alkies are us!", but after Googling "are us" -shopping and leafing through several pages of results, I have to say it's not exactly a commonplace idiomatic usage outside commercial contexts (that's to say, I didn't come across a single instance of anything 'non-commerical').
Grammatically speaking it's, well, not. You can say, for example, "We are human" in standard English, and "Human are we" in Yoda-speak, but "Human[s] are us" is never going to cut it.
Best Answer
Source: Merriam-Webster Dictionary