I think this may be an issue with exporting skeletal mesh to FBX from the UE4 editor. When importing the FBX character to 3ds Max (2013), the IK bones lose their constraints and do not affect the other bones at all. This doesn’t seem to be an issue when importing the FBX into Maya (2014).
Upon further investigation, it seems that the IK bones are actually not constrained to any of the skeleton bones. I’m just unsure what their function is if they don’t affect the character in any way. IK_Gun, IK_Hand L/R and IK_Foot L/R are their own separate hierarchy in the default rigging tools skeleton export.