艰 (jiān): Difficult, hard, arduous. This character sets the tone of struggle.
难 (nán): Also means difficult, but can imply a problem or disaster. It doubles down on the meaning of `艰`. Together, `艰难` (jiānnán) is a common word for “difficult” or “hard.”
险 (xiǎn): Danger, risk, or a perilous place (like a steep cliff). This character introduces the element of peril.
阻 (zǔ): To obstruct, block, or hinder. This implies a physical or metaphorical barrier in the way. Together, `险阻` (xiǎnzǔ) means “dangers and obstructions.”
By combining these four characters, the idiom creates a comprehensive and powerful image of a situation filled with every conceivable type of challenge: it's arduous, problematic, dangerous, and blocked.