TechPowerUp came across photos that show what's beneath the heatspreader of AMD's new Bristol Ridge Socket AM4 processors. Head over to the source for more photo of the die as well as the pin-grid on the underside of the CPU. Based on the photos, it looks like AMD is using liquid metal thermal compound between the die and the IHS.
Overclocker Nam Dae Won, with access to a couple of socket AM4 chips (most likely 7th generation A-series "Bristol Ridge" APUs), de-lidded the chips, revealing a large rectangular die. AMD is using high-quality TIM between the die and the IHS, which could either be solder or liquid metal.