We have several S4810 switches added to our NPM and NCM, and I don't recall any major issues adding them. Validate you are using SNMP v2c. I have a read only string set for NPM polling, and a read/write string set for advanced control from NCM. I only have ACL setup for the RW community string. Below is the SNMP config from one of our switches:
snmp-server community XXXX ro
snmp-server community YYYY rw snmp-rw-acl
snmp-server contact ME
snmp-server location Our Data Center
What version firmware is on your S4810 switches?