after a quick verification on Cisco Website, it seems that the 4215 is end-of-life and sales so it would be better to upgrade the hardware before putting in production a device which will face a lack of support.
In regards to the design you have two options in my view :
1. INLINE MODE with inline vlan pair or vlan group in the case where your servers are in different vlan in DMZ
2. PROMISCUOUS MODE with shun depending of the type of router or switch that you have
I added promiscuous mode in regards to your statement about availability.
In regard to your environment the options for availability are :
1. hardware and/or software bypass, I'm not so sure if the harware bypass card is supported by the 4215 device
2. install a second IPS or use a cable between the switches where IPS is connected. In this case you'll need spanning-tree configuration on ports
I'm not convinced about about the efficiency of the sensing link outside the network, may be for anomaly detection purposes ?
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...