Mismatched Pull Up Resistors
By today's standards 4MHz is by no means a high clock speed. Most often pull up resistor values are not that critical, people often use 10k for address or data lines without even thinking about it (just like 100nF is used for decoupling). Using for instance 6k8 instead of 10k won't do any harm. Just make sure the pull up is on the trace itself, and not on a stub.