I'm just curious how big Ubuntu's rootfs would be because Nexus 4 has no microsd slot and only 16gb of memory.
On the 8GB Nexus 4 (mako) I use for development that I've recently reflashed:
So if you subtract the 1.7GB of data I have in my home directory, I have about 4.1GB free space after the system image. So for the 16GB model, you'd probably have around 12GB free to play with.
Those numbers may change a bit as the OS is developed.
According to http://en.wikipedia.org/wiki/Ubuntu_Touch#Hardware_requirements , ubuntu touch on tablet requires at least 8GB of space.
Unfortunately, I cannot offer any further insight.