Question : Cisco ASA 5510 interface config

I have a Cisco ASA 5510 and it has 4 interface ports and a management port. I am trying to configure the interfaces but somehow it stops at 2 so this command:
interface Ethernet0/3
Gives this error:
                                ^
ERROR: % Invalid input detected at '^' marker.

How is it that i can only configure 3 interfaces and management and not 4 and management?

Answer : Cisco ASA 5510 interface config

you dont have the license to use the third interface
3: Ext: Not licensed        : irq 9