gistfile1.js

var fs = require('fs');

var files = ['file1.txt', 'file2.txt', 'file3.txt', 'file4.txt'];
var fileData = {};

files.forEach(funcion (name, index) {
    fs.readFile(nam, function (err, data) {
      if (err) {
        console.log('Error:', err);
      }
      fileData[name] = data;
      if (index === files.length - 1) {
          processFiles();
      }
    });
});

function processFiles() {
    // work with files
}