Linux kernel 4 has been release and can be installed and upgraded. Whilst the Ubuntu repository is still currently on version < 3.19. Kernel 4 does have a number of useful and features and bug fixes.
To install and upgrade your current kernel version you first need to download the deb packages from kernel.ubuntu.com. To install a new kernel version you will need to download 3 packages.
Downloads can be found in the kernel.ubuntu.com main steam site.
When choosing your download I recommend not downloading anything which is an RC (release candidate) unless you need to for a specific reason (like bug fixes). The latest kernel releases will be towards the bottom of the page.
Once you have downloaded your three files as stated above proceed to install them using:
sudo dpkg -i ~<path-to-deb> # install in the order stated above
Once installed you will need to update grub to use the newest installed version of the kernel. This is done by running:
Now, restart your machine and in terminal type
uname -r your kernel version should now be updated.