Gen IV isn't really harder than earlier generations were before the disassemblies/decompilations; many parts of it are actually
easier because of the way the DS ROM files are structured. All of the data tables (Pokemon, move, item and item stats for example) are in individual files that can be resized without rearranging the ROM, so you don't need to worry about pointers for most basic edits. The biggest challenge is the lack of research compared to the earlier games; you may have to do some actual hacking/research yourself occasionally.
I'd suggest getting started with some simple editing first, there are lots of tools
here (though many of them are garbage or unfinished) that can help familiarize you with the way the ROM works. You'll also want
this if you're hacking Platinum, or
this if you're hacking HGSS. Platinum also has a partial disassembly
here which is very useful. If you want to edit move stats I'd suggest using
my Nightmare module, which is better than every other Gen IV move editor I've seen.
You'll also need to know that none of the script editors for the DS games work properly, and there are no full tutorials for using them either. I'm currently working on fixing and eventually adding to SDSME, but it''ll a while before I get it everything working properly.
Hope that helps, I'll be happy to answer any questions you have if I can.