4 dbwr processes on 1 5400RPM disk = performance sucks
4 dbwr processes on stiped 15KRPM FCAL disks = good performance.

If you have 4 dbwr processes configured and they are all pumping data to the same disk, there will be disk contention. If you have 4 dbwr processes configured and the CPU is spinning at around 80-90% and the disks are < 50% busy, you are doing pretty good.