When firing a cluster of four model rocket engines it is very critical to get all four engines started before liftoff. A failed ignition on one or more engine means the rocket will not achieve proper altitude before parachute deployment, or worse will not fly straight up.
In this test, there was 0.17s (170 ms) from the start of the first engine to the ignition of the other three engines, which appear to have all started at the same time.
I think that trying to find igniters that are more "matched" (similar amounts of material) would improve ignition synchronization.
On the slow motion, you may note the motors' initial high impulse thrust for the first half-second or so, meant to get the rocket off the pad, before it backs off to a lower impulse thrust just before the coasting phase begins.
I'm calling this test a 'success', as all 4 engines ignited before the ignition clips were blown away from the test bed by the high impulse thrust.
The engines were all Estes A3-4T and were glued together prior to this test.