Four symmetric cores - all A15s. I don't think NVidia has ever done big.LITTLE even though they had a fifth core with the same architecture but different process/gate sizes for lower power in some previous Tegra chips. But you can see from them all having the same max frequency that all four cores are the same.
The companion core of the ARMv7 TK1 is a low power Cortex-A15, like the grandparent comment says. This is different from ARM's big.LITTLE which uses different microarchitectures for the two clusters, i.e. Cortex-A15 + Cortex-A7 or Cortex-A53 + Cortex-A57.
Core size isn't just a matter of the number of gates, but also their size. One important tool for reaching high frequencies in chips is to have gates that drive more capacities load be wider and therefore higher power. But if your goal is to have an architecturally identical core that's lower power one of the tools you have to accomplish that is to not do that. There's probably other ways to shrink a core at the cost of performance, as well.
Sorry, but that looks more like an artist's vision of a die shot than an actual die shot. The TRM is pretty clear that it's an A15: http://i.imgur.com/Fvh2KVT.png
