var str = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer vitae 1234 justo.";

var newStr = "";
var num = "";
var isNum = false;

for (var i = 0; i < str.length; i++) {
    if (str[i] >= '0' && str[i] <= '9') {
        num += str[i];
        isNum = true;
    } else {
        if (isNum && (num.length === 3 || num.length === 4)) {
            newStr += num + " ";
        }
        num = "";
        isNum = false;
    }
}

if (isNum && (num.length === 3 || num.length === 4)) {
    newStr += num + " ";
}

alert(newStr.trim());