Here is the tip, if somebody has the same issue (waiting for recovery mode) :

Phone on "recovery mode".

Terminal :
sudo adb devices

If ‘no permission’ :
sudo adb kill-server
sudo adb start-server

@advocatux It doesnt work.

But is doesnt matter because I UPGRADED MY MX4 TO UBPORTS !!! Finally !

As i thought, the main problem was the buggy recovery mode.
I tried :
sudo adb devices
75HABLM26VFN no permissions (verify udev rules); see [http://developer.android.com/tools/device.html]

(note : all these commands were made while phone on "recovery mode")

This "no permissions" was the key.

Then i did :
slayne@slayne-PcAsus:~$ sudo adb kill-server
slayne@slayne-PcAsus:~$ sudo adb start-server

• daemon not running. starting it now on port 5037 *
• daemon started successfully *
  slayne@slayne-PcAsus:~$ adb devices
  List of devices attached
  75HABLM26VFN recovery

That "recovery" instead of "no permissions" seemed pretty good.

I retried one of the last commands i did yesterday :
slayne@slayne-PcAsus:~$ sudo ubuntu-device-flash --server=https://system-image.ubports.com touch --device=arale --channel=ubports-touch/16.04/rc

2018/08/21 09:56:51 Device is |arale|
2018/08/21 09:56:51 Flashing version 9 from ubports-touch/16.04/rc channel and server https://system-image.ubports.com to device arale
2018/08/21 09:56:52 Start pushing /home/slayne/.cache/ubuntuimages/pool/keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz to device
2018/08/21 09:56:52 Start pushing /home/slayne/.cache/ubuntuimages/ubports-touch/16.04/rc/arale/version-9.tar.xz to device
2018/08/21 09:56:52 Done pushing /home/slayne/.cache/ubuntuimages/pool/keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz to device
2018/08/21 09:56:52 Start pushing /home/slayne/.cache/ubuntuimages/gpg/image-master.tar.xz to device
2018/08/21 09:56:52 Done pushing /home/slayne/.cache/ubuntuimages/ubports-touch/16.04/rc/arale/version-9.tar.xz to device
2018/08/21 09:56:52 Start pushing /home/slayne/.cache/ubuntuimages/gpg/image-signing.tar.xz to device
2018/08/21 09:56:52 Done pushing /home/slayne/.cache/ubuntuimages/gpg/image-master.tar.xz to device
2018/08/21 09:56:52 Done pushing /home/slayne/.cache/ubuntuimages/gpg/image-signing.tar.xz to device
2018/08/21 09:57:03 Start pushing /home/slayne/.cache/ubuntuimages/pool/device-c746103f205781974a7b89aff29cdf3dd450879f119f672d3a6f62b67ed783c5.tar.xz to device
2018/08/21 09:57:03 Start pushing /home/slayne/.cache/ubuntuimages/pool/ubports-dfcbe0416dc5dddaa398eff83a60d883c0d693dd1b1958e2fd9b61a02771e4ee.tar.xz to device
2018/08/21 09:57:35 Done pushing /home/slayne/.cache/ubuntuimages/pool/device-c746103f205781974a7b89aff29cdf3dd450879f119f672d3a6f62b67ed783c5.tar.xz to device
2018/08/21 09:59:27 Done pushing /home/slayne/.cache/ubuntuimages/pool/ubports-dfcbe0416dc5dddaa398eff83a60d883c0d693dd1b1958e2fd9b61a02771e4ee.tar.xz to device
2018/08/21 09:59:27 Created ubuntu_command: /home/slayne/.cache/ubuntuimages/ubuntu_commands324087129
2018/08/21 09:59:27 Rebooting into recovery to flash

AND IT WORKED !!

Thanks everydoby for your time and your help. Now im in.