Ask in the PPro forum and provide details - a lot more details. System info, type of files, storage used and so on....
Moving this discussion to the Premiere Pro CS5, CS5.5, & CS6 forum.
Fairly standard problem, and the "usual" solution is just what you are doing... continue to use CS4 for old projects
CS4 is a 32bit program, CS5 and later was completely rewritten to be 64bit... so what happens inside the program is also different, from somewhat to a lot
CS5 and later added MANY new options... and even opening a CS5 project in CS6 may not work properly
Post a screen shot of your CS4 project opened in CS6. This should work just fine.