The XDA developers have managed to install Android L on both these old devices.
For a Nexus 4:
Check this thread for a Nexus 4.
For a Nexus 7(version 2012):
This thread if you have an old Nexus 7.
Warning: These are beta versions and have many defects. A lot of things have not been tested yet on a basic level and the apps are simply not working as of yet.You can find a list of problems and issues by clicking this link.
Related: Nexus 5 Review
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