### Clue: Something very funny about 4's topping, a fermented dairy product (4,5)
### Answer: SOUR CREAM 
 ### Explanation: Definition: a fermented dairy product

**SCREAM** ("something very funny") *about ***OUR **("4's topping")
 
*This is an unusual clue as the 4 relates not to the solution to 4dn, but to the clue.