To further Atheus' point, the most compact way I can think to do it is: "" + 3 + 4 + 1 + 2. A bit ugly though, as the empty string is meaningless and distracting.
You can concatenate an integer (or pretty much anything) to a string and it will be coerced. Non-primitive types get their toString() method called; primitive types are probably replaced with calls like Integer.toString().