First, find the total number of points Carl needs to earn across all the tests by multiplying his desired average by the number of tests: 85 * 4 = <<85*4=340>>340 marks.
Then subtract all Carl's previous test scores to find the last score he needs to earn: 340 - 80 - 75 - 90 = <<340-80-75-90=95>>95 marks.
#### 95