I prefer to think of this problem as a coin rotating around a spot; i.e. the 'big' circle has a radius of zero.
It's easy to see that the 'small' circle will rotate once, even though it's travelled a circumference on 0cm.
Then, if the 'big' circle has any additional circumference (in this case 4cm), the 'small' circle will rotate that once, then travel the additional circumference.
Food for thought
I prefer to think of this problem as a coin rotating around a spot; i.e. the 'big' circle has a radius of zero.
It's easy to see that the 'small' circle will rotate once, even though it's travelled a circumference on 0cm.
Then, if the 'big' circle has any additional circumference (in this case 4cm), the 'small' circle will rotate that once, then travel the additional circumference.
I hope this makes sense!