Tony, if you use Photoshop, you will need to find the version of Adobe Camera Raw (ACR) that will open the MKIV files, BUT it depends on what version of PS you have as some of the earlier versions may not support the mkIV.
If you are shooting raw, you really need a converter, Lightroom & PS are the most popular then, depending if you are mac or PC, there are others too. Canon DPP actually reads the raw files pretty good and it is free.
If you just want a raw viewer, I use Faststone (PC only)