The name Flat-4 is derived from the description "Full-range 2-Element 4-Way Effect."
According to the company website this 4-way effect is as follows:
Placement of opposed elements reduces mechanical vibration due to anti-resonance.
Twin element parallel drive improves sound pressure sensitivity of low to middle frequency sound.
External auditory meatus tuning technology using phase correction tubes dramatically improves middle frequency sound.
Deletion of acoustic resistance and employment of small elements dynamically enhances ultra-high frequency sound and realizes wide sound propagation.
These technological innovations result in a rather unique looking earphone. The Flat-4 uses Comply foam tips. Currently available in Japan only, priced at 33,000 JPY.