This problem isn't really related to 4:3, task on bugtracker here: PA#3631 So it's present on 1366x768 too.
The game has a minimum screen resolution of 1280x720(16:9). So it appears that 4:3 monitors are not supported. - http://support.uberent.com/kb/faq.php?id=41
Yeah, it looks like the width of the inner container was set to be the same as the outer one, but padding wasn't accounted for. It's really not a big deal.
All I have to say is Uber has made decision not to support 4:3 monitors as per this knowledge base article.
They set the minimum resolution to 1280x720 as a consequence the minimum aspect ratio is set to 16:9 for the simple fact that 720p has an 16:9 aspect ratio. Edit: Not officially supported but yes it can be done through mod.