If you install the Web Client before you install Federated Search 10.4, Federated Search thinks that it is already installed and shows you the following window:

What's really going on is that the Document Preview Service is already installed, but not Federated Search itself. By clicking next it then gives you the ability to Modify, Repair, or Remove. If you click on Modify you can then select to install Federated Search 10.4.

I don't believe it should be doing that and that it should behave much the same way as when the OCR engine is already installed and you install another product that wants to install it as well.