node.js: multithread, using more than one core

I am trying to divide a task in node.js onto several cores (using a i5 I have 4 cores available). So far every explanation I found was to cryptic for me (especially the ones talking about servers, which I have no idea of). Can someone show me on the simple example below how I can split the task onto several cores?

Example: I just want to split the task, so that each core runs one of the loops. How do I do that?

var fs = require('fs');
var greater = fs.createWriteStream('greater.txt');
var smaller = fs.createWriteStream('smaller.txt');

for (var i=0; i<10000; i++){
    var input = Math.random()*100;
    if (input > 50){
        greater.write(input + '\r\n');
    }
}

for (var i=0; i<10000; i++){
    var input = Math.random()*100;
    if (input < 50){
        smaller.write(input + '\r\n');
    }
}    

greater.end();
smaller.end();