Upgrading from codewarrior v4 to v9

[Pat]

I am trying to convert current in-house custom designed applications
from v4 to v9. I consistently receive the following error messages:
US.prefix.h cannot be opened, palmOS.h cannot be opened, and
palmOSglue.h cannot be opened. I've checked my compile access path and
it appears to be correct. I also created a test application in v9 and
had no problem. However, once I tried to compile the older version 4
application, I get those error messages. I've also tried to target the
version 4 application to mirror the test application. My question is:
what I am doing wrong? Is version 4 just too old and has no forward
compatibility with version 9? Could someone give me some insight as to
what I am doing wrong or what I need to know about upgrading from
version 4 to version 9.