HI,
I am working on v9.1 FP 2
want to install FP 4
OS is AIX version 5.3
HACMP version 5.4

I am not finding any document, even from IBM how to install FP4 in HACMP.
if any one have any document or approach to follow kindly share with me.

There's nothing special about it. You install the fixpak on the standby node, switch over, migrate the database to the new fixpak level if it's required by the fixpak, install the fixpak on the service node, and switch over back to the original configuration.

There are many redbooks covering this in detail: search for "DB2 high availability" on www.redbooks.ibm.com