That's because a is a number, not a string. What you probably want to do is something like this:

var val = Math.floor(1000 + Math.random() * 9000);
console.log(val);

• Math.random() will generate a floating point number in the range [0, 1) (this is not a typo, it is standard mathematical notation to show that 1 is excluded from the range).
• Multiplying by 9000 results in a range of [0, 9000).
• Adding 1000 results in a range of [1000, 10000).
• Flooring chops off the decimal value to give you an integer. Note that it does not round.

General Case

If you want to generate an integer in the range [x, y), you can use the following code:

Math.floor(x + (y - x) * Math.random());

This will generate 4-digit random number (0000-9999) using substring:

var seq = (Math.floor(Math.random() * 10000) + 10000).toString().substring(1);
console.log(seq);

I adapted Balajis to make it immutable and functional.

Because this doesn't use math you can use alphanumeric, emojis, very long pins etc

const getRandomPin = (chars, len)=>[...Array(len)].map(
   (i)=>chars[Math.floor(Math.random()*chars.length)]
).join('');


//use it like this
getRandomPin('0123456789',4);