As for singlethread something in the 4.5 Ghz-4.7 Ghz range might not be too far-fetched for the upper Tigerlake-U models.
I'm leaning towards 4.3GHz being top clock for the U. Whatever it can achieve at base clocks change totally when looking at peak clocks, when looking at 4-plus GHz frequencies.*
I expect similar to Kabylake. A 28W model has 3.5GHz base clocks and 4GHz Turbo clocks. We could see 3.3-3.5GHz for base but I expect peak to remain similar at 4.3GHz(maybe 4.2GHz for 15W).
What's more important is that its IccMax came down while achieving higher frequencies. So they have more control over the chip, and I bet its V/F curves are less steep, alowing higher end of the frequency to use less power compared to Icelake. Icelake's 10nm seems great at low frequencies. But that quickly reverses at higher speeds.
*At such frequencies its almost nothing to do with capabilities of the process and design and almost entirely up to maturity, tweaking, and jacking up thermals. Had Intel ended 14nm reign with Skylake I doubt we'd have seen above 4.5GHz.
@Cardyak Further addressing your points, recently engineers(such as from AMD) said expect future processes to decrease in clocks.