You may try restarting it after shutting it down for a 1/2 hour or so and see if the x's go away. If it's
dirty inside the case you may blow it off so it's able to cool properly. While the case is off make sure (Unplug First) the connections are all tight and the boards are securely tightened down as well as the power connectors.
* If it's the stock power supply that is usually the culprit, it's under powered and not supplying enough juice to the hashing boards. Do you have a power meter to see if it's not pulling close to the 1400w it's rated. Mine wouldn't pull more than a 1000w from the wall and appeared that one board was bad. New power supply plugged in resolved what I thought was a bad board.
I was lucky and never hit the bad S4 PSU. But sadly a lot hit them with S4's. Testing it is not a bad idea at all.
My suggestion on cleaning is if you have owned it for a while it's not a bad idea at all. When I sold my S4's they did not look bad on outside, but I opened it and lot of dust in a not to dusty area. I would turn off as suggested and open it up look fan's first as they tend to hold dust. You can unscrew metal cover if really bad and get to blades easy. Then focus on heat sinks on blades you will need a can of air for sure on this project.
If nothing else you have a nice clean S4 after it.
