Like its RF 600mm counterpart and as expected, this lens turns in outstanding results.
Despite the Canon RF 400mm F2.8 L IS USM Lens and Canon EF 400mm f/2.8L IS III USM Lens being nearly identical, including a shared optical design, one opportunity for the RF 400 to differentiate itself was optical performance with extenders. More specifically, RF extenders vs. EF extenders.
There is some improvement with the RF extenders in place, and the 1.4x comparison at f/4.5 shows slightly more improvement.
Can the RF 400mm F2.8 lens and 1.4x extender substitute for an RF 600mm F4? Perhaps. Keeping in mind that 560mm is not 600mm, check out the Canon RF 400mm F2.8 L IS USM Lens vs. Canon RF 600mm F4 L IS USM Lens comparison. Then remember that the 600mm lens does not have f/2.8 available.