When you reverse the digits in a certain two-digit number, you decrease its value by 18. What is the number is the sum of its digits is 4?

It is $13$

Explanation:

Let $x$ and $\left(4 - x\right)$ represent the unit and tens digits of this certain two-digit number

10*(4-x)+x=10*x+(4-x)-18=> 40-10x+x=10x+4-x-18=> 40+18-4=10x+10x-2x=> 54=18x=>x=3

Hence the unit digit is 3 the tens unit is 1.So the number is 13.

Check :$31 - 13 = 18$