How do I mount my Samsung galaxy s4 on ubuntu?
Open a terminal in ubuntu(ctrl+alt+t) and type this commands:

sudo add-apt-repository ppa:langdalepl/gvfs-mtp
sudo apt-get update


Then, launch Software Updater (previously known as Update Manager) and install the available updates.
Afther you pdate everything restart pc.

if you want to revert back before making any changes:

sudo ppa-purge ppa:langdalepl/gvfs-mtp


Next time, I think you should ask this is askubuntu.com since it is more of a linux specific problem then android.