Literally: four – suffering – eight – suffering
Alternately: Deep distress, terrible hardship, anguish, malaise. The very bad feels.
Notes: You might think, as I did, that this phrase is nothing more than a doubling for effect, but it turns out to have esoteric Buddhist roots and carry an implication closer to “every possible kind of suffering.”
First, the 四苦 form another yojijukugo in their own right: 生老病死, shou rou byou shi, or “birth, age, sickness, death.” These are the four kinds of suffering that come automatically with living and can’t be avoided. To get eight, you add four more: the pain of being separated from those you love, the pain of meeting someone you hate, the pain of not getting something that you want, and the pain caused by the five skandha… for some reason they left out the pain caused by unwanted recursion.
One folk etymology puns on ku also being a reading of 九, “nine.” It claims that shiku is 4×9 (36) and hakku is 8×9 (72), and that adding them together gives you 108, a significant number in various Buddhist traditions. This connection seems to have been added after the fact, though.