[buffers]
bypass_cache=0
If this value is set to 1 DP4 will open files in such a way as the operating system is unable to cache writes. You might want to do this if you suspect the operating system is failing to save data to disk when it should. Effectively this means all files are treated as though they were opened with the VF_SYNC flag. Write caching by DP4 is unaffected. The operating system read cache is not bypassed.
From 4.622 setting bypass_cache=2 disables all operating system caching and DP4 entirely takes over responsibility for caching database file access. If your databases are small this is unlikely to be beneficial, but on larger databases it can improve performance significantly (by 50% or even 75%). If you choose to do this you may need to tune all the [buffers] settings carefully if you want to get the best possible performance.
On Windows based operating systems, using this setting forces DP4 to write files in sector sized amounts. This means that a file may occasionally contain close to a sector of binary zeros you were not expecting at the end of the file. When the file is closed DP4 briefly re-opens the file and truncates it, removing the spurious zero bytes. However, in the event of a system crash DP4 will not have been able to do this.