Goldman Sachs warns that oil prices could fall below $40 a barrel in an 'extreme' scenario.Brent crude oil hits a four-year low at $63.90 per barrel.The global oil market is experiencing volatility due to the Trump administration's trade war escalation and pushback from economies such as China.Goldman Sachs analysts anticipate Brent crude to be at $55 per barrel by December next year in the bank's base-case outlook.