Here is an issue to be aware of for those upgrading to or installing a new ArcGIS License Managers for 10.4. I did this to support prerelease testing.
In my case I installed the new license manager on a fresh Windows 7 Professional (64 bit) install. I then updated the services.txt file as required to open the firewall to allow traffic between the FLEX LM manager and other computers on the network. I was then able to successfully checkout/checkin licenses from my ArcGIS Desktop 10.3.1 client and the ArcGIS 10.4 License Manager.
I then tested the Borrow license feature for ArcGIS Desktop Basic (client machine). This succeded. However, when attempting to return the license the ArcGIS Adminstrator (on the Desktop machine) would time out after about 20 seconds and return an error message. The Borrow/Return screen would then refresh and shown the license had not been returned.
