I saw an ad for an Iwata Hi-Line 4003 and was curious as to how it differed from their Hi-Line 4100? I did not see much information on the details of the 4003.

There may be a build difference, but I doubt it. It appears that the packaging was changed with the H4003 being the earlier release.

It looked like the head was diffrent in the photo's, more like a Micron head.

There was a change in the nozzles/body at one point. They made the threads larger so it was less likely to strip or become damaged.

Not sure if the numbers reflect that change or not.

The HP-C Hi-Line doesn’t have the ability to take a separate head assembly like the Micron and the change in the nozzle threads isn’t visible without removing the nozzle itself. Of course, it’s entirely possible that someone put the incorrect picture with the the text you saw.