Maybe you have to install the extension for the Agent 4.5 again. I recommend to remove the extension and to reinstall it after you have checked, that the Agent 4.5 is listed under your software repository under the "current branch".
I have also added the agent version 4.5 to my ePO 4.o today but in the following order:
1.) Checked in the new package under Software=>Check in Package
2.) Checked in the new extensions under Configuration=>Extension=>Install Extension
I didn't remove the "old" Agent in Version 4.0 before I checked in the new packages and I see now both versions in the drop down menu (under deploy agents).
So if my first recommendations doesn't help try to remove the package and extension of version 4.5 and install version 4.0 again before you install the new one again.
If everything seems to be fine then, so that you can see both versions in the drop down menu, start the deploy the new agent. If this process is finished you can safely remove the old agent from your repository.
I don't know if you ever figured this out, but the problem might be with your version of ePO. I know that you have to be on a certain build in order to see version 4.5 of the agent. We're on ePO 4.0 Build 1333 and I'm able to see the agent.