Install Hotfix Volume name

From TMSH Reference Guide:

Volume names are in the format HDX.Y, CFX.Y, or MDX.Y, where X is the hard drive index (HDX), compact flash index (CFX), or RAID index (MDX) (onsystems that support RAID), and Y is the volume number on that drive.

So first volume on the first hard drive would be HD1.1. The second volume HD1.2, etc

When installing hot-fixes 4, Do I need to install hotfix 1,2,and 3 first?

You need only install HF4. Each hotfix includes all previous hotfixes. See SOL13123