Raistmer,
If image support only was for the streaming model, then it could be supported on HD4XXX. However, there are many issues with the OpenCL spec that 4XXX does not comply with. For example, on 8XX hardware there is support for scatter based typed surfaces(write_only images), but this does not exist on 7XX hardware, so write_only images can not be supported in hardware, and that is required for 'image' support.