I've never used an HP ze4440, so I don't know if they come pre-set to boot from USB devices. However, in any computer, there are certain things that must be done to make them capable of booting from USB devices.
First and foremost, the source device (in this case, a USB flash drive) must be formatted with a FAT32 file system in order to be recognized by the BIOS as a bootable device. You can check the drive's file system by opening My Computer/Computer, right clicking the drive, and selecting Properties. The file system should be displayed on the General tab.
Second, Legacy USB Support must be enabled within the BIOS to allow the BIOS to recognize properly formatted USB devices as bootable.
And third, you must also set the Boot Priority/Order within the BIOS to include USB devices. A PC's Boot Priority is typically set to optical drives and hard disks only by default. Mind you, USB Devices must be placed ahead of hard disk, but can be ahead of or follow CD/DVD-ROM. (Just be sure only the correct one is either installed or has bootable media on it.) Once those 3 tasks are complete, you should be able to boot from any USB storage device containing a bootable disk image.
It's entirely possible some of the settings required to do this are already enabled on your system, but you'll need to enter the BIOS to be certain. (The BIOS is often referred to as Setup, and most often accessed by pressing either the F2 or F10 key during the system's POST display.)