Note, that if you have now 96MB of shared buffers,
you are sill on the rather low side with your 4GB of memory.
Here are some old recommendations from postgres tuning:
* Start at 4MB (512) for a workstation
* Medium size data set and 256-512MB available RAM: 16-32MB (2048-4096)
* Large dataset and lots of available RAM (1-4GB): 64-256MB (8192-32768)
Use "ipcs -m" and "ipcs -ml" on Linux to find memory usage and current limits.