Type 4 echoes represent a rare but distinct spectral signature of auroral electrojet backscatter during times of large electric fields and strong two-stream-generated ion acoustic turbulence. On the basis of many bistatic CW Doppler radar observations made over the past 15 years the great majority of type 4 echoes have a composite spectrum showing two main features, a broadened type 1 component peaking at elevated ion acoustic velocities and the type 4 narrow component at roughly double the ion acoustic speed. The most important property is the large phase velocities, mostly seen in the 900 to 1100 m/s range, associated with irregularities propagating in a narrow azimuthal cone along the destabilizing Hall current. In the present paper it is argued tht the existing interpretation, in which it is hypothesized that the large Doppler velocities are simply from two stream waves originating in regions of highly elevated electron temperature, cannot explain why the observed phase velocities of type 4 waves are mostly seen at preferential values near 1000 m/s and why these waves are highly directional. On the basis of the existing evidence we conclude that there is not yet satisfactory explanation for the type 4 echoes. The fact that type 4 waves have frequencies of about two times the frequency of the simultaneously observed ion acoustic waves led us to suggest the possibility of a resonant three-wave coupling interaction process which might be in operation under some appropriate conditions in the plasma. At present this is only a preliminary proposition which would be investigated further in a future study. ¿American Geophysical Union 1991