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.
This appeared to be a timeout issue, fixes such as adding the FLEXLM_TIMEOUT environment variable on the client (with values as high as 20 seconds) were tried but failed to improve the situation. On the host (license manager) machine we tried adding the computer names into the hosts table.
During a ping attempt while testing connections between the two computers we noticed that on the host machine it was returning the IPv6 address when pinged, when the same machine was pinged from the client machine it returned and IPv4 address. This turned out to be the problem. To fix the connection issue it was necessary to change the "priority" of the IP addresses. In Windows 7 and later IPv6 is the prefered/first addressed returned. To change the priority I used the Prefer IPv4 over IPv6 in prefix policies FixIt tool available from Microsoft for KB92985 (https://support.microsoft.com/en-us/kb/929852).
Disabling the IPv6 support on the network card itself worked as well but was not a viable longterm solution on our network.
Hope this helps someone.