Tegra 4 may be one of the most powerful mobile chips this year, but it's probably not one of the most efficient, which is why you're only going to see it in tablets, and it seems it's also quite pricey. Some OEM's like HP, Asus and Toshiba have signed up for it, but it's likely it won't be used in high volume products this year. That means very little revenue in return for Nvidia.
Nvidia has also lost the contract for the next-gen Nexus 7, which was won by Qualcomm instead, and that will be a huge blow to the company's Tegra sales this year, because last year Nexus 7 accounted for the majority of the sales of Tegra 3-powered devices.
If this rumor that Tegra 4 chips are also too expensive from compared to other chips from competitors is true, then the high power consumption may not be the only reason why Google chose not to use Tegra 4 for the next-gen Nexus 7. The Nexus 7 is a $200 device that's worth significantly more, but because Google is subsidized part of the cost, you get to pay less for it. But if the Tegra 4 was too expensive, that may also be one of the reason why they didn't go with it.
Another reason could be that Tegra 4 doesn't even have an integrated LTE modem in it, like most of Qualcomm's chips, too, and I'm quite certain Google intends to sell the new Nexus 7 with LTE on board, too (the higher priced version, at least). Nvidia will only offer integrated LTE for their true "smartphone chip", the Tegra 4i, which is not based on Cortex A15, but on a newer, more powerful version of Cortex A9, and comes with a higher clock speed. But even that chip isn't supposed to arrive until later this year, not in time for the launch of the next-gen Nexus 7'.
So all of these factors make Google's decision to go with Qualcomm instead, pretty easy, and I'm sure it's why many other OEM's chose Qualcomm over Nvidia this year, too, for most of their products. Nvidia will need to really step it up next year with Tegra 5 in terms of efficiency, performance, and even price, if they want to get back in the game in a significant way.