Re: I'm unable to download anything new or get updates on my Nexus 4.
I'm assuming you man from the Play Store? Does it give you an error or you just can't open the store?
To clear cache you need to go into Settings, find the Application Manager, scroll to the ALL tab, find the Play Store and there you have the option to clear cache. You can also try to unisntall updates from it, reboot, and let it re-update itself.
Re: I'm unable to download anything new or get updates on my Nexus 4.
A few more details would be helpful. What are you trying to download?
As SpookDroid mentioned, go to Settings>Apps>All, select Google Play Store, and Clear Cache/Data, then Force Stop. Do the same for Download Manager. Make sure Download Manager isn't Disabled as well.