File 4 is $AttrDef, which contains the attribute names for all attributes used on the volume. Currently, only the well-known attributes should appear. Each record in this file has A0 bytes 
Offset(length)    Description
0(80)             Attribute name (Unicode)
80(8)             Attribute ID
88(8)             Flags
90(8)             Minimum allowed size
98(8)             Maximum allowed size
The exact meaning of the flags is still not clear. Most likely, bit 7 means 'may be non-resident', and bit 1 'indexable'. Bit 6 is also in use, all other flags are zero for the available attributes.