```ts
export function lastFour(number: string): string {
  return number.length >= 4 ? number.slice(-4) : number;
}

export function maskCard(number: string): string {
  if (number === null || number === undefined) {
    throw new Error("Input must be a non-null value");
  }

  const stars = "*".repeat(number.length - 4);
  return stars + number.slice(-4);
}
```