Let see...
ARC4+ is the premium version with better color temp., more even anodizing, and perhaps a little brighter.
ARC4(seconds) is suppose to be ARC4+ that either didn't make the full brightness, have some cosmetic issue or software issues such as certain function not working
ARC4 might be a little dimmer than the ARC4+ and probably vary slightly in color temp. It might even have grainy anodizing as well when compared to the ARC4+ but other than that, it funtions perfectly
That what I think anyway. I'm sure someone will chime in soon
~Stephen