Good morning, I have a Nexus 4 and I wonder if Canonical or someone has some tool to do a dual-boot as GRUB for PC ... allowing boot of Android or Ubuntu on the same device.
It is in principle possible to install a dual-boot system for Android and Ubuntu on your Nexus 4. However, this is not intended to be used by regular users. Rather, it is meant as a possibility for developers and early testers to develop and test Ubuntu for smartphones without having to renounce to their normal Android system. Quoting from the Ubuntu Wiki,
Ubuntu Dual Boot Installer is provided as a tech preview for developers who want to run Ubuntu and Android on a single device. It is not intended to be used by regular users, neither at this point nor as its ultimate goal. Those developers installing it should be familiar with the Ubuntu and Android partition layouts and should also feel at home with manually flashing partitions in case something goes wrong.
If you still want to do this, find the full instructions here: https://wiki.ubuntu.com/Touch/DualBootInstallation
Please note that Ubuntu for smartphones itself - even without such things as dual-boot - is not yet aimed at end users yet. It is still in a stage of development and testing. Quoting from the official Ubuntu for Phone install guide,
Ubuntu for phones is intended to be used for development and evaluation purposes only. It is an early release that can potentially brick your device. It does not provide all of the features and services of a retail phone and cannot replace your current handset. This is the first release of Ubuntu for phones and it will continue to evolve quickly. Please read the disclaimer as well. This process will delete all data from the device. Restoring Android will not restore this data.
So, if you want to contribute to Ubuntu and help out testing, and know your way around smartphones, Android and Ubuntu, go ahead. However, if you expect a smoothly running system without flaws, I would suggest for you to wait a little.
As already written in Malte Skoruppa's answer, a tool exists and has it's own Wiki page.
If you know how to unlock, backup and flash a (factory) image to your phone by using Android development tools like
adb, and this is what you do anyway, you can give it whirl.
From what I have seen on my device, this particular implementation seems to replace the recovery image with Ubuntu Touch on the device. So usually your phone boots into Android unless you tell it to boot into recovery by key combo at boot or via commands/app, which boots Ubuntu Touch.
I am using Multirom to kind of dual boot. It is an android app. You can also easily install Ubuntu Touch through this app but you should root Nexus 4(or install CyanogenMod)