That is because of the license restriction for your 515e pix. I guess you should be having Pix license with dmz bundle. You can upgrade with an upgraded license version or you can try to upgrade to ASA OS for the pix and check if this issue gets resolved.
You can refer the below document for more information.
This is normal based on your appliance license. It looks like you have the Restricted Software license (see bold part below).
• The Cisco PIX 515E Restricted (PIX 515E-R) model provides an excellent value for organizations looking for robust Cisco PIX Security Appliance services with minimal interface density and VPN throughput requirements. It includes 64 MB of RAM, two 10/100 Fast Ethernet interfaces, and support for one additional 10/100 Fast Ethernet interface.
• The PIX 515E Unrestricted (PIX 515E-UR) model extends the capabilities of the family with support for stateful failover, additional LAN interfaces, and increased VPN throughput via integrated hardware-based VPN acceleration. It includes an integrated VAC or VAC+ hardware VPN accelerator, 128 MB of RAM, two 10/100 Fast Ethernet interfaces, and support for up to four additional 10/100 Fast Ethernet interfaces. The Cisco PIX 515E-UR also adds the ability to share state information with a secondary Cisco PIX Security Appliance (either in an Active/Active or Active/Standby deployment model) for resilient network protection."
So, how can I change to Unrestriced Sofeware License mode so that i can use 4 more Fast Ether interface???