The Etymotic ER-4 MicroPro series is the company's top-end in-ear headphone line. Currently, the series consists of four models, each of which retail for $299.
According to the company, here's how the differences break down:
The Etymotic MicroPro ER-4B is "for binaural recording monitoring and used in professional settings."
The Etymotic MicroPro ER-4S is the company's "stereo monitoring earphone and geared to those listening through a powered recording or 'front of house' monitoring board."
The Etymotic MicroPro ER-4P is the most mainstream of the bunch, designed for those using a portable audio device as their primary audio source.
The Etymotic MicroPro ER-4PT is the latest version of the 4P. Etymotic says it's made for the professional traveler who uses his or her earphones on planes as well as professionally.
To be clear, the ER-4Ps and ER-4PTs are essentially the same products, but they're packaged differently. The ER-4PT has different adaptors for plane use and also comes with a Channel Matching Compliance Graph that states to the owner that his or her ER-4PT has passed all accuracy tests and each channel has been hand matched and certified within 2 dB over a defined audio spectrum.
Check out CNET's full review of the Etymotic ER-4PT, which should provide a good overview on how all four models perform.
(If you're still confused, check out Etymotic's website for a more detailed explanation on the ER-4 MicroPro model line.)