A rectangle with adjacent sides \(6\text{ cm}\) and \(4\text{ cm}\) satisfies the lengths.

A parallelogram with the same side lengths and an interior angle of \(60^\circ\) also satisfies them, but is not a rectangle. The side lengths alone do not require right angles.

Answer

No. A \(6\text{ cm}\) by \(4\text{ cm}\) rectangle and a slanted parallelogram with those side lengths are both possible.