Let O and A represent 0 and 4 repectivelyon the number line l as shown in the figure.

Step 1.

Draw a right angle triangle OBA, ∠A = 90

Now, OA = 4 and AB = 1

Using Pythagorus Theorem,

      OB² = OA² + AB²

=> OB² = 4² + 1²

=> OB² = 16 + 1

=> OB² = 17

=> OB = √17

Step 2.

Now, taking O as centre and OB as radius, draw an arc to cut the number line l at C.

So, OC = √17