Well, I made a trip to BestBuy, PW1 in hand. I held the two devices side by side and walked through the majority of the functionality. I tethered my PW1 to my cell phone, and the store model was on their wifi. I downloaded a sample of the book on the demo unit.
Speed... almost identical. I could hardly discern a difference when opening books, changing pages, changing font settings, looking up words, selecting text, browsing the online store, etc, etc. In some instances my PW1 actually finished first when touching with thumb and pinky simultaneously.
Lighting... the lighting on the PW4 was definitely more consistent. The screen was definitely more white. That said, I kind of liked the slightly yellowish lighting and screen color of my device better. Maybe it's from being used to it, but it was warmer -- something that people keep asking for on the Kindle -- warmth adjustment, so I'll take that as a win for the PW1.
Screen quality... the PW4 screen definitely had more depth to its images. I compared some of the images in the book and one in particular really jumped out at me as having more apparent shades of gray. I took a picture of it.
Wow... that's quite a difference, not something that I see mentioned in most reviews. Fine detail in images was slightly better on the PW4. In addition, the PW1 screen had to do full-screen flashing refreshes more often, and did leave more faint artifacts now and then when opening images/menus/etc.
Glare... I actually tried to induce glare from the lights in the store, and with my phone flash... it turns out that the PW4 actually looked BETTER than the PW1 in that regards, but only slightly. (funny, my picture shows the glare on my PW1 screen, but not the PW4 screen -- but that could be the angle)
Anyhow... after about 40 minutes of playing with them both side by side I decided that the differences weren't significant enough to cause me to upgrade, and there were actually some things I like better about my old one (especially the old official case, which is nicer than the new official case)
So, I left the store empty-handed. I think I've scratched my itch and it went away. :-) Maybe the PW5 will tempt me again next year and then I'll be glad that I held out for some new feature.