Different Launchers for Different Workspaces?
Solution 1:
It took a few years, but now you can have a launcher with diffent pinned applications in different workspaces with Unity LauncherSwitcher. (Thanks to webupd8.org for making me aware of it.)
Run these commands to install it:
sudo add-apt-repository ppa:vlijm/lswitcher
sudo apt-get update
sudo apt-get install lswitcher
Solution 2:
Unity does not have 4 launchers so you can not have different launchers for different viewports (yes I used viewports, not workspaces).
Unity Workspaces Switcher
uses Compiz version of viewports so basically you have a launcher that activates a split screen based on the vertical and horizontal amount of viewports. This is not the same as workspaces in Gnome 2. This is actually 1 workspace with 1,2,4 or more viewports.