Oracle has recently released Oracle Patch Set update (PSU 2) along with CPU October 2009. As informed earlier , PSU are released quarterly along with CPU patches.
If you are running 10.2.0.4 RAC database, you need to apply following patches
1) Patch 8833280 (10.2.0.4.2 PSU 2) for Oracle Database 10.2.0.4
2) Patch 8436582 (10.2.0.4 CRS Bundle Patch #4) for Oracle Clusterware 10.2.0.4 CRS PSU2 patch contains fixes for bugs fixed in Bundle patch 4
3) Patch 8705958 (10.2.0.4 CRS PSU 2 ) for Oracle Clusterware 10.2.0.4
Critical Patch Update (CPU) are released each quarter and contain security fixes identified by Oracle. Oracle has recently introduced, Patch Set Update (PSUs) which are proactive cumulative patches containing recommended bug fixes that are released on a regular and predictable schedule (it also contains Security fixes part of CPU). PSUs are on the same quarterly schedule as the Critical Patch Updates (CPU), specifically the Tuesday closest to the 15th of January, April, July, and October.
It is recommended to apply latest Database Patchet (aleady on 10.2.0.4) along with latest Patch Set Update (PSU2) to Oracle Database as this helps to fix the critical bugs and also serve as a new baseline version for reporting issues to Oracle.
In case of RAC, you also need to apply CRS Patch Bundle to fix the CRS issues identified by Oracle.
Please note that Patch Set Updates can be applied on the base release version or on any earlier Patch Set Update. For example, 10.2.0.4.2 can be applied on 10.2.0.4.1 and 10.2.0.4.0 . Once a Patch Set Update has been applied, the recommended way to get future security content is to apply subsequent Patch Set Updates. Reverting from an applied Patch Set Update back to the Critical Patch Update, while technically possible, requires significant time and effort, and is not advised.
To know further, please read following My Oracle Support articles
Note 405820.1 - 10.2.0.X CRS Bundle Patch Information
Note 854428.1 - Intro to Patch Set Updates (PSU)
Note 8833280.8 lists down the bugs fixed as part of PSU2.