Have i worked it out correct = 25132.74 milesSince Earth is not a perfect sphere of 4000 miles radius, I could logically say anything I want and my statement would be "true"; but that wouldn't be what you're asking. You mean, "If Earth were a perfect sphere of 4,000 miles radius what would be its circumference in miles?"
Do you know the formula for the circumference of a circle? It's \(\displaystyle C = 2\pi r\).
Replace \(\displaystyle r\) with 4000, and do the calculation.