Just curious, why would it be easier to test two engines? The crew of flight engineers already have their hands full testing one engine, why complicate things by adding more engines?
During flight test the job of the other 3 engines is to keep the aircraft flying. This allows the crew of flight test engineers to focus on the one test engine. On the first couple flights, the test engine may not even be used at critical phases (i.e. during takeoff) until it demonstrates that it can operate safely in the air.
Also, throughout an engine development test program, unexpected things can and will happen. That's the point of engine testing, you want to find problems and fix them before your customers start flying your engine. So no matter how much time and effort is put into engineering design before testing, engines will break. The goal is to find all the big issues during ground test and fix them before flight test. However, Murphy's Law still governs in the air, so if something can go wrong, it will. The flight crew needs the capability of shutting down the test engine if needed.