You do not need to purchase VMS-MC to update your sensors. VMS does run for a 90 day tiral peroid, if you do wish to download and try it out. VMS has been replaced with CSM (for the MC functionality) and MARS (for the SecMon functionality). VMS will still work with version 4.x and 5.x sensors (but not 6.x), and VMS SecMon will give you the reports your looking for.
The current version of sensor software (4.x) has not had any signature updates for about 18 months. If you have a CCO account, you can download version 5.1.7E1 upgrade (you put it on an FTP server and run the "upgrade ftp://user@/IPS-K9-5.1-7-E1.pkg" at the sensor's CLI). here: http://www.cisco.com/cgi-bin/tablebuild.pl/ips5
or download the IPS-K9-cd-1.1-a-5.1-7-E1.iso image and burn yourself a CD to reformat your 4235 (all current settings will be lost). here: http://www.cisco.com/cgi-bin/tablebuild.pl/ips5-system
You can also download the 6.0(4) version software the same ways as above, Both 5.x and 6.x contain current signature packs to the date they were released (6.0(4)a is slightly more current). All downloads: http://www.cisco.com/kobayashi/sw-center/ciscosecure/ids/crypto/index.shtml
good luck