Step 1: $OQ\perp PQ$ and $OR\perp PR$ (radius $\perp$ tangent), so $\angle OQP=\angle ORP=90^\circ$.
Step 2: In quadrilateral OQPR, angle sum $=360^\circ$:
$$\angle QOR=360^\circ-90^\circ-90^\circ-90^\circ=90^\circ$$
Step 3: Also $OQ=OR=4$ cm, so OQPR is a square of side 4 cm.
Step 4: OP is its diagonal:
$$OP=\sqrt{4^2+4^2}=\sqrt{32}=4\sqrt2\text{ cm}$$
Answer: (B) $4\sqrt{2}$ cm