What are M4 & T4 protocol of Hearing aid compatibility of GSM & CDMA model of iPhone 4? Saw this in Wikipedia (http://en.wikipedia.org/wiki/Iphone_4): 

Hearing aid compatibility


GSM
3G 850/1,900 MHz M4, T4
2G 850 MHz M3, T3
2G 1,900 MHz M2, T3

CDMA M4, T4


So what do M4, T4, M2, ...  mean?
If you see a M3, M4, T3 or T4 on the box then the phone has been designated as HAC compliant. 

About the T


  "T" refers to the phone's coupling ability and is intended for use
  with hearing aids in the telecoil mode. The higher the "T" number the
  more likely you will be able to use the phone with your hearing aid on
  the telecoil setting. A telecoil is a small device that is built into
  some hearing aids for use with the telephone as well as assistive
  listening devices. To use the telecoil, generally either the hearing
  aid is switched to the "T" position or a button on the hearing aid is
  pushed to select the telecoil program. The telecoil picks up magnetic
  fields generated by telephones and converts these fields into sound.
  Telecoils are particularly useful for telephone communication because
  they permit the volume control of a hearing aid to be turned up
  without creating feedback or "whistling," and background noise can be
  reduced especially when using cell phones in noisy places. That is why
  hearing aid phones are important. They give you the ability to control
  and process sound.


source: betterhearing.org

About the difference between M and T


  An "M" rating measures compatibility with acoustic hearing aids while
  a "T" rating measures compatibility with induction hearing aids. So
  the higher the number (e.g., M3, M4 etc.) the more compatible the
  handset. Generally, handsets rated "M3" or "T3" by ANSI or better are
  considered compliant with FCC requirements. Acoustic Hearing Aids
  M3-rating indicates the handset has satisfied the ANSI standard
  M4-rating indicates the handset has exceeded the ANSI standard
  
  Induction Hearing Aids T3-rating indicates the handset has satisfied
  the ANSI standard T4-rating indicates the handset has exceeded the
  ANSI standard


source: uscellular.com