Pure IPC comparisons, normalized clocks generation over generation.
The CPUs ability to clock is a secondary discussion. I don't expect Zen to have 4GHz base clocks, probably around 3.5, with turbo to 4, and reliable overclocking barely exceeding the turbo. And that's at the high end, with four cores.
Zen could be a decent upgrade for anyone who doesn't overclock, however, and is on a Sandy Bridge or older system. Considering moving from any i7 to any AMD CPU has been a general downgrade since time immemorial, I'd say that is a drastic improvement.
Zen will, by no means, defeat Intel.
I expect them to use SMT to even the playing field, price-wise, with Intel's i5s, and to add a couple more cores to compete i7s, and then have some eight cores that will compete with Intel's six core CPUs.