I have been working with an ASA5510 with simmilar behavior. It has 4 Ethernet ports (plus the Management Ethernet port) but only 3 of the Ethernet ports are available for configuration. I believe that it is a licensing issue. Under our license we have Inside, Outside, and DMZ and the fourth port is not available. I assume that with different licensig the fourth port becomes available.
The above is correct your ASA 5510 only has the Cisco ASA 5510 bundle base. Which only allows for 3 FE ports. Check out the below link for the descriptions of each of the bundles. From the ASDM on the main page on the license tab you can see what your are currently licensed for.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...