It's not unusual for mobile devices to run hot when heavily used, and large, high-res screens use a lot of power, which generates heat. But the Tab S 8.4 may have a more serious issue. The Russian web site Hi-Tech.Mail.Ru is reporting at least one case of this tablet running hot enough to melt the plastic back of the device.
Samsung is aware of the problem, and has stated that they are investigating, but that damage to the back cover is "neglible" and doesn't affect the functionality of the tablet. It is not clear if this is a widespread problem or limited to a small batch of tablets, nor if this affects only the Exynos-powered version. If you own a Tab S and have this problem, you should contact Samsung as soon as possible. Please let us know what Samsung says in the comments section, below.