Hi netlife,
Port security will allow only one device to operate on a port. It looks at the source MAC address of the packets coming in the port and, if they are not the same as the first packet it saw on that port (or you can give it a specific MAC addres yourself via the config) it thinks that someone has unplugged the device that was there and put a new device on instead, so it figures that there's a security breach and shuts down the port.
Since you've put the 1920 on, the 4006 starts to see multiple MACs on port 2/47 and shuts it down. That's completey normal behaviour and proves the port security is working.
You'll just have to remove port security on 2/47 and it'll work just fine after that.
Hope that helps,
Paul.