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

export function maskCard(number: string): string {
  if (typeof number !== 'string') throw new Error('Invalid input');
  const stars = '*'.repeat(number.length - 4);
  return `${stars}${number.slice(-4)}`;
}
```