Or M3...A Zen 3 core at peak spends 19W-20W. A Firestorm core at peak spends 5W. Ignoring the I/O die, apple cores are significantly more efficient at peak performance.
By the time Zen 4 comes out, it will be competing against M2.
Zen to Zen2 was ~2.5 years.
Zen2 to Zen3 was ~1.5 years
There MIGHT be a Zen3+ competing against the A15, but I'd expect for most of its lifetime, except perhaps for a few months, Zen4 will be competing against A16.