As most people had said before this series started, the magnitude of this series really meant it had to be 5 tests to be honest or at minimum 4 tests
Even if South Africa win at Lords (especially if its a tight win), who is to say England couldn't come back and draw or win the series?.
Looking back maybe the West Indies should have only had 2 tests or the Australian ODI series should not have happened (although that series was organized mainly to get gates with the EUROs and Olympics around).
However in an attempt to predict the results if England do manage to win the final test and draw the series, South Africa would only have themselves to blame and it would be fair to call them test match chockers, just like they are readily referred to as ODI chockers given their failures in major world-cup knockout matches.
S Africa could have been clear # 1 in everyone's mind if they had beaten India in India in 2008 or 2010. But they lost the final tests of both series after winning the 1st tests, so the ghost of those results will be put to the test @ Lord's.
For England if they lose, it won't be in the end of the world. All it would mean is that they have lost to a S Africa team which in theory is better than them.
England team would still be good enough to win in India later this year and the Ashes next year.