I can guarantee that the Cisco engineers are going to tell you to upgrade to 6.x. Beyond that, I would recommend you upgrade to 5.x since signature support stopped for 4.x some time ago. Odds are also that memory is being held for disk caching etc instead of being solely in use by the CIDS processes. Is it dropping packets or have you noticed performance issues?
What was meant by "dropping packets" was the sensor actually discarding packets instead of inspecting them
You can look for this on the command line by watching a show events. The sensor will display what the percentage of "dropped packets" are occurring if at all.
What happens is the IOS stores all of the packets inbound to its sensing interface in a temporary cache and queues them up for inspection. Since the cache has a finite size if it gets to large, the sensor simply rejects new input and "gets behind" in its inspection. What you see is a resource hit of the CPU and or memory pegged at nearly 100%.
You really should upgrade to 5.x as some of the newer engines in 5.x actually directly address many of the issues you will run into in 4.x, and more importantly signature support for 4.x ended quite some time ago, so you will not be able to detect the more recent worms/exploits/etc.
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...