How to downgrade Safari from 8.0.6 to 8.0.4? I am investigating a website bug that seems to only occur in Safari 8.0.4 (Yosemite 10.10.2), but I am on 8.0.6 (Yosemite 10.10.3) already.

Is there any way to downgrade Safari to the patchlevel I require that does not include restoring an old backup from Time Machine or reinstalling the OS?
For testing environments I recommend using Parallels or VMware. You could do an install of Yosemite and then only install the updates needed.

I usually create multiple virtual machines by creating a base VM, then copying it, doing a round of updates, then copy that one, then doing another round on the next copy, then repeat that process until I have all of the software versions I am looking for.

Keep in mind that some of the software you may want to use will require a separate license for each virtual machine.