This specifies the AFI, IDI, and preDSP to use when constructing an
 NSAP from a Phase IV address.  This is used when a Phase IV address is
 specified for a node identification or in an address tower.

 For example if the Phase IV address is "1.5", a Phase IV prefix value of:

     39:840:800AB738

 results in the following NSAP being used to represent the DECnet Phase IV
 address:

     39:840:800AB738-0001:AA-00-04-00-05-04:20

 If any Phase IV addresses require the use of some Phase IV prefix other
 than the value set using this field, they can be specified using a Phase
 IV address syntax that includes the Phase IV prefix.  This will override
 the default value specified in this form.  For example:

     1.5+39:840:800AB79

 See the MAIN_MENU PHASE_IV_PREFIXES topic for details about Phase IV
 prefix values.