Yeah man in one of my (decent) local tv shops they have a Sony 40W4500 (admittedly not the Z4500 but fairly similar) next to the Panasonic 42PZ800 and the Panny smashed it. The Panny's pictures were so sharp and clear - possibly one of the sharpest and clearest images I have seen and a possibly a credible rival to Pioneer's excellent offerings. The Sony looked good in isolation but when it was next to the Panny it paled in comparison. For example, on the menu screen of a Blu-ray, the letters on the Sony were a little blurred while on the Panny they were perfectly sharp and defined. The only problem with the Panny was that when movie content was being shown, the images looked unnaturally realistic
What I mean by this oxymoron is that the film ended up looking like a reality show or fly on the wall programme - realistic but not the same as the natural look of film. However I'm told this is due to the '24p true film' mode which can be switched off. Don't confuse this with the standard filmic frame-rate of 24fps which is not a 'mode' and it cannot be switched off. The '24p true film' mode is in addition to the tv's standard handling of 24fps material and alters the content by interpolating (a kind of mathematical educated guessing) - ie. adding - frames. God knows why they do this but some people might like it I suppose. Anyway, out of the Sony 40W4500 and the Panny 42PZ800 the Panny was clearly superior.
This doesn't really answer your question though seeing as you would like to know about the Sony 40Z4500. As for that tv, it is very good. It displays the typical Sony lcd picture - quite a cool colour balance and slightly unnatural (but quite stylish) look, with a tiny hint of (also stylish) filmic graininess - I think. Apparently it is an excellent tv where motion is concerned. In tests it was found to be able to resolve the full 1080p resolution during movement which is unique in an lcd. Usually they can only keep there 1080 lines intact when the image is fairly still. Because of this, and it's bright lcd picture, this Sony might be the best choice if you like playing games, or perhaps watching sport. And for movies maybe it should be the Panny that you go for - but remember to keep that 'true film' mode off.
What you really need to do is see the Sony 40Z4500 and the Panasonic 42PZ800 side by side with a variety of content; standard-def, high-def, sports, movies, games, freeview, sky, sky+hd, blu-ray, upscaled dvds, etc. Good luck.