I have installed automount and has indeed allowed me to mount ext4 partitions, but only as root, though.
What is output of sysctl vfs.usermount on Your system? I have it at '1' to allow user to mount.
About Thunar issue, I assumed that the DISPLAY would be :0.0, maybe its different in Your case (is it different?). I may modify the automount to read that variable dynamically.