Zen/+ itself can do >5GHz in both 14nm and 12nm iterations, it just needs cold to do it. Clockspeed here is not an architectural limitation, it has to do with the fact that these processes were meant for low power SoCs with the even more important drawback of these using a 20nm BEOL, this puts such speeds well out of these nodes' intended target. Zen's physical implementation on these probably isn't the best it can be either, too. It all adds to a ~4GHz wall for Zen and a ~4.3GHz wall for Zen+ at borderline dangerous voltages for longterm usage at ambient temperatures.
TSMC's 7nm should let Zen2 scale higher than ~4.3GHz as seen in 12nm+Zen+ at ambient temperatures, that's a given, especially if AMD has gone with the HP variant.
Expecting 4.5GHz tops from this transition to a one and a half node jump (leaving the 20nm BEOL behind) despite the ever decreasing clockspeed scaling is just being too damn negative IMO. ~5GHz is, after all, the current reasonable clockspeed limit under reasonable conditions until further advances are made in materials and finding ways to cheat physics out of unintended behaviors as we go forward.. this limit applies to both Intel and AMD, they're both fighting the same physics.
Sure, Intel got to 5GHz on *lake at ambient temperatures and safe voltages first (no, faildozer+32nm doesn't count) by fine tuning their 14nm process and physical implementation of Skylake to perfection by necessity. AMD can do the same (or get reasonably close to that number) with a more competent process available to them and more money to throw at the problem for the refinement and fine tuning required. They're not operating on a shoestring budget anymore as they did when designing Zen, there are more possibilities open for them to take.
5GHz would be ideal, yet I think it'll happen as 4.35GHz does on the 2700x via XFR2/PBO: high voltages to sustain that clockspeed for peak temporary ST performance in a way that's safe for long term usage, but not viable for safe manual OC/all core usage at such voltages as can be done with the 9900k, for example.