The following OID works on my routers, switches and also a 6509 with an MSFC and it produces the 5 min average utilisation of the CPU, it will probably work on a 4006 switch.
my_switch# sh proc cpu
CPU utilization for five seconds: 9%/8%; one minute: 10%; five minutes: 9%
PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process
$ snmpget my_switch 1.3.6.1.4.1.9.2.1.58.0
cisco.local.lsystem.avgBusy5.0 : INTEGER: 10
See the following link for details on the OID
http://tools.cisco.com/Support/SNMP/do/BrowseOID.do?objectInput=1.3.6.1.4.1.9.2.1.58&translate=Translate&submitValue=SUBMIT
Good Luck
Paddy