It is Duokan that is trying to use more space than you have available on a stock kindle firmware (no hacks installed, and not too many books). That is a Duokan problem.
Any halfway decent documentation should list the possible error messages that the program can display, with a list of possible causes, and a list of remedies to fix the problem. Doukan has been around long enough, and it is apparently being developed by a professional organization (i.e. paid programmers), so it SHOULD have decent documentation by now. Because it is a PROFESSIONAL program (which happens to be free for now while in development), it should have what you need to solve your problem (or at least provide technical feedback and support). Of course, there may be a slight language barrier issue.
What does the Duokan documentation tell you to do about your error message?
P.S. You may get more support in this forum if you post to a Duokan thread that is monitored by Duokan fans, but starting a new thread (like this one) is less likely to get noticed by those who COULD help you. WE can help you with KINDLE problems. The Duokan people can help you with DUOKAN problems. What you describe certainly looks like a DUOKAN problem, NOT a Kindle problem. Duokan may only install reliably on some localized versions of the K4, but not on yours. They need to test their program on a broader base of K4 versions, but they are still early in the K4 development cycle, so give them time. But at least contact them for assistance so that they CAN fix the problem, or the next version of Duokan may fail on your K4 as well...