The formula for circumference is: $${\mathtt{C}} = {\mathtt{\pi}}{\mathtt{\,\times\,}}{\mathtt{d}}$$ where d is the diameter of the circle.
Therefore, plug in 4 for d, and the circumference would equal $${\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{\pi}} = {\mathtt{12.566\: \!370\: \!614\: \!359\: \!173}}$$. You can write this either as 4 pi or round the decimal.
The formula for area of a circle is: $${\mathtt{A}} = {\mathtt{\pi}}{\mathtt{\,\times\,}}{{\mathtt{r}}}^{{\mathtt{2}}}$$ where r is the radius of the circle. Since radius is half of the diameter, the radius of this circle = 2
And 22 is still 4. So the area would be 4 pi as well, about 12.57.
The difference between the two values for circumference and area of this circle is their units. Area is in units squared.