Though official Catalyst software 13.1 is suitable for ati radeon hd4330, it can't be installed on Ubuntu 14.04 as it can't support Xorg >= 7.6 As I need proprietary drivers for trusty, I would like to know if there is a way to bypass this limitation ? (For example by fetching driver sources)

Here are some results from the terminal :

$ Xorg -version

X.Org X Server 1.15.1
Release Date: 2014-04-13
X Protocol Version 11, Revision 0
Build Operating System: Linux 3.2.0-37-generic x86_64 Ubuntu
Current Operating System: Linux bordeaux80 3.13.0-27-generic #50-Ubuntu SMP Thu May 15 18:06:16 UTC 2014 x86_64
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-3.13.0-27-generic root=UUID=4015e6f7-d11a-45fd-ac9b-5b6c7ab9eaa0 ro quiet splash vt.handoff=7
Build Date: 16 April 2014  01:36:29PM
xorg-server 2:1.15.1-0ubuntu2 (For technical support please see http://www.ubuntu.com/support) 
Current version of pixman: 0.30.2
    Before reporting problems, check http://wiki.x.org
    to make sure that you have the latest version.

$ xrandr

Screen 0: minimum 320 x 200, current 1366 x 768, maximum 8192 x 8192
LVDS connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 353mm x 198mm
   1366x768       60.0*+
   1280x720       59.9  
   1152x768       59.8  
   1024x768       59.9  
   800x600        59.9  
   848x480        59.7  
   720x480        59.7  
   640x480        59.4  
VGA-0 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)

$ uname -rp

3.13.0-27-generic x86_64

$ glxinfo | grep OpenGL

OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD RV710
OpenGL core profile version string: 3.1 (Core Profile) Mesa 10.1.0
OpenGL core profile shading language version string: 1.40
OpenGL core profile context flags: (none)
OpenGL core profile extensions:
OpenGL version string: 3.0 Mesa 10.1.0
OpenGL shading language version string: 1.30
OpenGL context flags: (none)
OpenGL extensions:

Regards