```ts
export function lastFour(number: string): string {
  if (!number) throw Error('Invalid input');
  return number.slice(-4);
}

export function maskCard(number: string): string {
  if (!number) throw Error('Invalid input');
  return Array.from(number).map((_, i) => i === number.length - 4 ? '*' : '#').join('');
}
```