logical deduction - Three boxes, Four questions, Three times not true, Once the truth

There are 3 boxes, only one of which contains a treasure.

The guardian will only answer you truthfully once, the other three times he will lie.

You may ask 4 YES or NO questions.

Which questions should you ask, to know which box contains the treasure?

Answer

Ask three times of box 1. If the answer changes, you know about box 1. If not ask of box 2. This question has a known truth/lie answer, so you know which box.