Kawai Synthesizers

I would focus on the LED contrast issue first...  the K4 may actually be working just fine, but you can't see anything on the display because the contrast is whack.  It is also possible you wired up the data connections incorrectly (even though you obviously got the backlight wired up okay).  So, double-check your work.
I did some experiments replacing the older-gen LCD screens on most of my gear with 16x2 displays -- and getting the contrast to work was always the most annoying part of the job.  I used a tiny 10K potentiometer (instead of a single resistor) so I could "dial-in" a working contrast after installation (much better to going back to re-solder the "correct" resistor, perhaps more than once :).
Ultimately though, I ended up using fancy new OLED screens for all my display "upgrades".  They look much better than even the most modern LCDs and don't need a contrast resistor!  OLED are much brighter and can be clearly read from almost any angle (in an OLED display, each pixel lights up individually and doesn't rely on a backlight or filtering through an LCD matrix).  IMO, OLED replacement displays are totally worth the cost ($20-40 instead of the $3 el cheapo LCDs you can get on ebay).
However...   I have yet to find an OLED screen that is compatible with my K4.  I've tried the Winstar and Electronic Assembly models (the EA models just seem to be re-branded Winstars).  Supposedly, all my devices use HD44780-compatible displays... but something about the K4 doesn't work with the Winstar (which is HD44780-compatible).  I've successfully installed OLED displays in a TX81Z, a Vintage Keys Plus, and an Alesis MMT-8.  I intend to upgrade my CZ-101 at some point as well.