OK:
To calculate the probability of engine failure on a 4 eng. aircraft compared to a twin:
A twin can suffer an engine failure senario in the following ways: Eng 1 can fail, Eng 2 can fail or Eng 1 & 2 can fail. For a 4 engine aircraft, the possible combinations becomes Eng 1, 2, 3, or 4. However, account needs to be taken of 2 engines failing (12 possible combinations), 3 engines failing (4 possible combinations) & all 4 engines failing.
This is why a 4 engine aeroplane has more that twice the chance of engine failure than a twin - assuming that each engine has the same probability of failure..