it also depends on your configuration.
Are your servers running VCS ? if yes are there parallel or failover services ? or its just a simple SF installation.
Are your servers also running Oracle RAC for example ? is replication setup to a DR site, etc ?
You would need to go step by step .. first would need to upgrade from 5.0 to 5.1, then apply 5.1SP1 & last step would be to apply 5.1SP1RP4.
You need to read the install guides & release notes for each product to find the steps to upgrade.
To go to 5.1
Sorry, I have been absent for a long time on the forums.
Finally yes it is possible, by the use of the base_path option of the installmr script.
First thing is to be positioned in the most advanced version software folder, in my case 6.1.1 (this is the maintenance release), then from there invoke the installmr script with the base_path option giving it the path to the 6.1.0 software folder (this is the base product). the script will import the necessary installation code.
worked perfectly for me, especially using a rolling upgrade approach.