It is very possible that you will have a color shift in your prints. The BCI 6 cartridges are to have a different ink formulation than the new BCI 8 cartridges used in your 4200.
This is all that I have read. I believe that BCI 3 cartridge from previous models might have the same ink as the large black in your printer.
I have also read that Canon only really changed the Magenta formulation, but nothing to show proof of that.
You also might want to check with the manufacture of the compatibles that you will be using, they might have some info on their website.