ı thınk the problem is
confıguratıon>>server settings >>reprository packages and there is no selected
if ı select yes can ı ınstall agent 4.0 and agent 4.5 same time???
If you choose "Yes" you will be able to check different package versions into different EPO Branches (i.e. Evaluation and Previous) I am pretty sure that you can only have one Agent version in each branch at any given time.