```ts
export function lastFour(number: string): string {
  if (typeof number !== 'string' || !number) {
    throw new Error('Invalid argument');
  }

  return number.length <= 4 ? number : number.slice(-4);
}

export function maskCard(number: string): string {
  if (typeof number !== 'string' || !number) {
    throw new Error('Invalid argument');
  }

  let masked = '';
  for (let i = 0; i < number.length; i++) {
    masked += '*';
  }
  return masked;
}
```