Each pool in your configuration .tcl file includes the number of connections allowed for that pool. You should have three pools for an OpenACS 4 installation. I think we set the default to five connections per pool so that would be fifteen connections per OpenACS instance, or sixty for four instances.
PG defaults to a maximum of a 100 open connections - you can change the configuration file in your PG install directory (i.e. /usr/local/pgsql/data I think by default if you install from source) to allow fewer or more. You can also limit your OpenACS pools to one or two connections at a time, too ... this just causes things to stall while waiting for a connection.
You're not really seeing that many connections, though ... maybe you should cut your buffer pools down from 7600 to give enough space for PG to open up more backends if you want to run four instances.
Or use AOLserver 4's virtual server capability to run all five out of one instance, you can do more resource sharing (at the cost of having all virtual servers shutdown/restart together) this way than when running a separate AOLserver process for each site. The latter's more convenient, the former more conservative of resource use.
