Let me start off with a disclaimer: I've never used REXX/400.
Now, let me give you a smart assed answer: There is no difference between MF REXX and REXX/400.
But seriously, I took a look at the first documentation I found on REXX/400, http://publib.boulder.ibm.com/iseries/v5r2/ic2924/books/c4157280.pdf, and there are no differences that I could find as far as the REXX language.
Of course there are many differences in the way that you interface with the operating system, and they seem to be documented quite well.
I'd guess that the reason you haven't gotten an answer from someone who's knowledgeable in REXX/400 is because there are way more differences between say Z/OS and AS/400 OS than can be detailed reasonably in an answer.
Have a look at the documentation, if you haven't already, and if you are still having problems with doing something in REXX/400, like reading a file for example, come back with a more specific question, including what you want to do, maybe how you would have done it on a mainframe, and what you've tried so far. I'm sure you'll get a little more help with that approach.
Best of luck,