The way way to solve this problem is to write an equation. "4 more" means that you are adding 4. "twice a number" means that you are multiplying a variable (lets name the number x) by 2.
Your equation would be
2x+4=8
The first thing you do is subtract 4 from both sides
2x=4
Finally, divide by two to find out what the unknown number
x=4
The number is 4
The equation is 2x+4=8