Treat certain WiFi networks as mobile/cellular I plan to be putting a Nexus 7 (will be rooted) in my car and tethering off my Nexus 4 (rooted). However I only get 1.5GB data per month on my mobile and don't want it to download my Spotify playlists, update apps etc whilst using my phones WiFi.

I'd like it to treat it as a mobile network and know that Android has the Mobile hotspots option but as of testing it did absolutely nothing AFAIK.

Open to pretty much all options.
If you go to Settings > Data usage and open the menu, then select Mobile hotspots you can select Wi-Fi networks that should be treated as mobile hotspots.

This should restrict the data usage when connected to these networks.

 
Data Usage and Mobile Hotspot screens (click images for larger variants)