Are LaunchAgents supported by OS X 10.4?
I tested them on 10.5, 10.6 and 10.7, and it worked well.
launchd was introduced in Mac OS X 10.4 "Tiger", and so 10.4 does support LaunchAgents.
Whether that version supports the features you need is another question. It's not really possible to give a better answer without knowing more details about your particular problem.