how can i install mac os x 10.4 an a external hardrive

Boot the Mac from the 10.4 installation disk, confirm with Disk Utility that the external drive is formatted as Mac OS X Extended (Journaled) with the appropriate partition type (GUID for an Intel Mac, Apple Partition Map for a PowerPC Mac), select the external hard drive as the disk on which to install, and install.

You posted in the Mac OS X 10.7 Lion forum, by the way. You'll usually get the quickest and most applicable responses if you ask in the forum for the product or service about which you're asking.

Regards.

FW will be far faster than USB.