Why does iphoto only open 4 images in the external editor? I am using Photoshop as the external editor, but when I select a series of photos (more than 4) within iPhoto and choose "open in external editor" only the first four of the selected open. It's pretty annoying because I don't want to have to keep going back and forth between the programs to see what i have open in Photoshop. I just want to select (holding control and selecting) different images to then edit outside of the program.

Is this a setting i can adjust? What possible reason would there be for this limitation?
I believe this is to protect against User error - the kind of thing that happens when an inexperienced user doesn't realise that s/he has every photo in their Library selected and then opts to send to the external editor. 

No, there is no setting to adjust. Nor have I ever found a way to hack the limitation.