My tests to find out whether it is better to guess questions whose answers you don't know, the marking scheme being:
+4 marks for correct answer
-1 mark for wrong answer
0 marks for skipped questions

Final Result:
68.36% chance of gaining marks
31.64% chance of losing marks

Some interesting observations:

The chance for gaining marks decreased to ~66% when the number of questions attempted is one more than a multiple of 4

The chance for gaining marks increased to ~71.8% when the number of questions attempted is two more than a multiple of 4

The chance for gaining marks decreased to ~53.9%, the chance for no net marks became 22.5% and the chance for losing marks decreased to 23.5% when the number of questions attempted is three more than a multiple of 4

The final result above only applies when number of questions attempted is a multiple of 4