If I were doing it, I'd create a global variable called sequence.
I'd give each painting's button it's own instance variable called
number, and assign them a number 1 through 4. (Mona -> 1, Supper -> 2, etc...)
I'd wrap the following in a group:
When a player jumps on a button, I would append the number of the button to the sequence variable. (i.e. they jumped on the Mona Lisa's button, so I would do System: Set Sequence to Sequence & Button.number)
Then, I would check to see if the sequence was not empty (sequence <> ""), I would then check if the sequence equaled the correct sequence to unlock the level. (sequence = "1324") If it does, do your unlock and turn this group off so it doesn't accidental trigger again if they jump on the buttons.
If it doesn't equal the full sequence, I would then check to see if it equals something other than the correct partial sequences (sequence != "1" and sequence != "13" and sequence != "132"). If it doesn't equal one of those, I'd turn on the red light and clear the current sequence.
<img src="http://www.infinitepossibilitygames.com/demos/sequence/sequence.png" border="0" />