What number goes in the missing space?

So to get from 4 to 8 you multiply by 2. To get from 8 to 16 you multiply by 2. So for the missing number, 10 times 2=20. The missing number should be 20. To check our work, 20 times 2 equals 40. So the missing number is 20. Hope this helps! ;)

21, i'm pretty sure it's right lol