Round off 4252 to one significant figure

The rounded result is 4,000. If you just write "4,000", though, it's not clear whether there are 1, 2, 3 or 4 significant figures, since trailing zeroes could be significant (or not). For that reason, scientific notation is often used. 4 × 10³ has one significant digit, 4.0 × 10³ has two, 4.00 × 10³ has three, and so on.

So, use either 4,000 or 4 × 10³, depending on what your teacher expects.