there is a socket in the unit that appears to be for a memory card. Unfortunately its non-standard. Back when the Fastpath 5 came out Shiva offered an upgrade for Fastpath 4 owners that added 256k of RAM, the 5.1 PROMs, and a fan. Looks like the unit got sent in to be upgraded by Shiva, not the end user.
You may be able to run K-STAR 9.1.2 on a FP4 and the 5.1 PROMs. 9.2's Readme explicitly says it needs 512k of RAM. There really is no benefit for the average home user to run the newer versions though since they only add tunneling capabilities. K-STAR 8.1.1 works fine for basic MacIP use. I have a configuration guide for the Fastpath 4 all typed up. I just have to proofread it.
You may be able to run K-STAR 9.1.2 on a FP4 and the 5.1 PROMs. 9.2's Readme explicitly says it needs 512k of RAM. There really is no benefit for the average home user to run the newer versions though since they only add tunneling capabilities. K-STAR 8.1.1 works fine for basic MacIP use. I have a configuration guide for the Fastpath 4 all typed up. I just have to proofread it.