Found a working binary for pngcrush on kindle on the web. Just in case some else needs it, there you go:
http://arm.koji.fedoraproject.org/ko...?buildID=11465
I downloaded the armv5tel binary rpm, opened it in 7-Zip (on W7), extracted the pngcrush executable and copied it to the K4 - working fine. Caution, newer versions didn't work for me, required a newer libpng than I had on my device. Going back to 1.6.10 seemed to be the easiest option.
So, thanks again to you guys for your valuable work and ideas!