*PEEP* Wrong!
Seperate mixers are also used in MIXING situations! Not all recording units have a builtin mixer. In fact, the better recorders don't have one. These are standalone expensive units. And you combine them with an even more expensive mixer...
The 4 tracks with built in (analog) mixers that I know, don't really have good mixer sections. But most of them do have an output for each track, so you can hook up a better mixer, with better EQ, insert compressors etc, more aux...
A mixer is just a thing with alot of inputs and alot of outputs, mainly used to get all the inputs on just 2 outputs, in a musical asjustable way. Now that was nicely explained, right?