=LOOKUP("C",{"a","b","c","d";1,2,3,4}) looks up "C" in the first row of the array, finds the largest value that is less than or equal to it, and then returns the value in the last row that's in the same column.
Input the formula: =LOOKUP("C",{"a","b","c","d";1,2,3,4}). Press Enter to get the result.