> Unless they felt the need to claim the quad-core bullet point
There you go.
There you go.
a) Wait for a quad-core Krait. Given HTC's close relationship with Qualcomm, they likely know it won't be for a while
b) Wait for a Tegra-compatible LTE chip, which hasn't been proven in the market yet. Still late to market, and with parts of questionable integrity.
c) Fall back on the dual-core chip and ship now. Lose a bullet point in the US market, but actually be on the market.
When the Tegra 2 showed up, it was not generally considered suitable for phones; the one found in a few phones is a slightly different model to the original. Also, the Tegra 2 was quite flawed; it lacked NEON, in particular, hurting software compatibility.
It's also possible that supplies of the S4 are restricted; it's a 28nm part, and TSMC's rollout of their 28nm process hasn't gone as smoothly as they might like. The Tegra 3 is 40nm, which is now a mainstream process.