I have flown a 6 a few times with no issues and when I get ready to fly the 4 I will go get a bunch of time in a 6 (because there are some available) which is a very similar airplane, same engine, wing, etc. Then I will conduct the first flight at high power to break in the engine. So I won't be doing any training or circuits, just doing what the engine needs up high and checking systems.
I think if the airplane and pilot are properly prepared for the first flight it should be a non-event. But preparation of both is critical. That's how it is at the aircraft company I work for - 1st flight is by competent crew with all systems checked out and all ground testing signed off. If you look at the accidents on 1st flights invariably something is missing in the prep. Not 100%, but in the vast majority of cases. Once could still have an engine failure due to an assembly error i.e. not fuel delivery or ignition, but again, that should be addressed in the test planning i.e. decision speed to commit to the takeoff, available fields if it quits on climbout (in front of you only), get to altitude immediately. The window for failure causing an off airport landing should be small. You can never get the risk to 0, but how low you get it is a function of your care in prepping the pilot (whoever it is) and the airplane.