Why was flash image over exposed?

I was practacing some off camera flash work and was shooting after dusk. My ambient light reading was something like 1/60, f/1.0, ISO 4000. with a canon 580 ex in manual mode at 1/128 power, my light reading was something like 1/60, f/4.5, ISO 4000. However, my image was over exposed. I dropped the ISO on my camera down to 1600, then the image was properly exposed.

I was using a Sekonic L-308S as a light meter (and now understand why a backlit LCD screen is important, which that model lacks) that is usually right on. Why would my image be so over exposed?

I might be misunderstanding, but your meter told you to use 1/60, 4.5 at iso 4000, but you chose to open up to 1.0 (your ambient) anyway? That would do it, or am I misunderstanding?

Wait, I get it. You wanted the ambient to be what it was, and the flash to "fill". You can't go any lower than 1/128th power, so the flash must have been too close?

Your question is short on information. Post the image and tell us your camera settings.