RPG/400

Note that

RPG/400 can do just about anything that earlier versions of RPG such as RPG III could do and a whole lot more,
while RPG 4 can do just about anything that RPG/400 could do and a whole lot more.

New stuff as of RPG/400

This is from the perspective of someone moving from RPG on S/36 SSP to RPG on the 400. Some of these capabilities may have existed with RPG on S/38.

Test a field for various kinds of validity before processing it
External File Definition Support. Previously we just had Internal File Definitions.
Massive new support for various kinds of Data Structures and Data Areas, which also can be Externally described
Open and Close files
Key Lists for access to indexed files
Expansion to the controls on accessing a file where
Many new structured programming capabilities in conjunction with IFxx
Refer to indicators using *IN## *ON or *OFF
Concatenation and Substringing
Scan a file to locate some character string, and record where it is (byte count sideways)
CALL a sub-program and be-called, with parameters passed between them
Output an interactive screen to the users, then read it back in and act on the data provided