Do they refer to the specific a/r? If so which is which, and how do I tell which is which. Are they marked?

 

TO4B and TO4E are names of different compressor covers. TO4B has a 2.625" inlet and a 1.75" outlet, TO4E has a 3" inlet and 2" outlet. The bigger TO4S has a 4" inlet and 2.5" outlet. This does not designate the size of the turbo, you can have a large turbo with a TO4B compressor cover that will outpower a TO4E turbo, but more often than not the TO4E housing will be accompanied by a larger wheel.