This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
What is exactly your question - what is the difference between what it does and what you expected it to do?
Did you expect a compile error because it isn't clear if the add() and subtract() methods in class Four are implementations of the interface methods or of the methods in the abstract class?
The answer to that is: There doesn't need to be a compiler error because there is no ambiguity - the methods add() and subtract() in class Four are implementations of the interface methods and the abstract class methods at the same time.
There is indeed no relationship between interface I1 and abstract class AbstractFourOne, but there doesn't need to be a relationship.