JSFiddle - React, Tailwind, and code Playground

JavaScript

var str = 'this is an example string.';

var arr = str.split(/\b/g);
var i, j;
for(i = 0; i < arr.length; i++){
    if(/^\w{4,}$/.test(arr[i])){
        var temp = [];
        var end = arr[i].length - 1;
        for(j = 1; j < end; j++)
            temp.push(arr[i][j]);
        arr[i] = arr[i][0] + shuffle(temp).join('') + arr[i][end];
    }
}
str = arr.join('');

document.write(str);

function shuffle(o){
    for(var j, x, i = o.length; i; j = parseInt(Math.random() * i), x = o[--i], o[i] = o[j], o[j] = x);
    return o;
};