They haven't lost any due to engine failure

The previous three were tank failures. This one appears to be a failure with the Ground Support Equipment. Speculation is that the hoses that feed liquid methane to the rocket separated after the engine fire. All that free propellant caught fire and destroyed the vehicle.

The important part of the failures is that they've had three failures in four months. The rate of improvement is impressive. They just destroyed SN4 and SN5 is already ready to put on the test stand (thought the test stand will need to be rebuilt). The failures are setting them back weeks rather than years.