Using a colour HP LaserJet 2550L here with no problems.
The TurboPrint driver for this model only supports greyscale, but it doesn't worry me as I nearly always send it PostScript outputs directly from FinalWriter, PageStream, IBrowse, etc.
I'm not familiar with the 4500 but I understand it supports PosctScript as well. Just make your application print to a PostScript file and then "copy output.ps PAR:". It should handle the colour printing correctly.
You can do a print preview using GhostScript to view the PostScript output before printing.
