"Your disc is currently unreadable" is this a very bad sign?

[Dodece]

Four possible solutions.

1. Use a can of compressed air to spray out the disc tray. Follow the instructions.
2. Fully power down your console unplug and leave unplugged for a few minutes.
3. When you start the game switch the audio track for the game then switch back. 4. Flip the discs back to three then reinsert disc four. Then quickly save game.

The compressed air can blow out any dust screwing with the laser simple enough, and you should always have a can for your high end electronics anyway. To deal with dust buildup.

Powering down the console is not simply a gag. Powering the console down should help to clear conflicting data out of the systems memory. Until you fully power down your console by unplugging it the data can just lie there. Screwing with you.

The last two issues involve region coding. The 360 contains regions specific data for every region. The region coding on disc four may be faulty. Which means its trying to run a different region then the consoles setting. Flipping the disc could reestablish the correct region coding, or the flipping of the language track can do the same.

Anyway you should try these four options. The last three are easy enough to do, and will cost you nothing. They are all basically software resolutions. The forth is one you should be considering regardless. High end hardware needs to be periodically cleaned anyway.