G.SHDSL devices use two parameters for line configuration: PAM and BASE RATE. While  G.SHDSL Economy and Etherlink II modems only know two PAM modes (16 and 32), Etherlink IV  works with PAM modes from 4 to 128. Although the PAM mode does not directly affect the transmission speed, it is, however, crucial to it. The speed is directly calculated from  the base rate number and depending on the PAM mode, only certain base rates are possible.

PAM Modus G.SHDSL.Economy Etherlink II Etherlink IV
4 - - 2-39
8 - - 3-79
16 3-60 3-60 4-119
32 12-89 12-89 4-159
64 - - 2-199
128 - - 4-238

The transmission speed is calculated by multiplying the base rate number with 64. The result is in kbit/s. At PAM 128 and base rate 238, Etherlink IV thus achieves 15,232  kbit/s; at PAM 32 and base rate 89, Etherlink II still achieves 5,696 kbit/s. You get the nominal values by adding 8Kb overhead for the transmission.
Depending on the device, the highest speed is only achieved under ideal line conditions (length, thickness, insulation, interferences, connections). Lower speeds, however, are  possible. Typically, the base rate is reduced. For lines with poor line conditions, a different PAM mode may improve the line performance.
Mentally, you can compare the relationship between PAM and base rate to a train going from A to B. You want to transport as many persons as possible in the shortest possible  time. The PAM mode determines the type, the base rate the number of coaches. Depending on the coach type, only a certain number of coaches can be attached. The line conditions  reduce the choice of coach types further.
The beauty of Etherlink IV is that you can choose from a wide range of coach types. The small PAM modes represent coaches suitable for poor line conditions. They occupy  different band widths and are highly immune to malfunctions and interferences.
Imagine, for example, a DSL line with several connections, plenty of crosstalk and interferences over a distance of 10 kilometers. For Etherlink II, you would have to choose a  small base rate of 20-25 at PAM 16 for a speed of 1.5 Mbit/s. For Etherlink IV you could select smaller PAM modes for a good and stable transmission that is 20 to 25% faster.  PAM 4 at a base rate of 25-30 already amounts to almost 2Mbit/s. Depending on the lines used, the values in the field differ from those in the examples.