There’s an old stretcher here.

There’s an old stretcher here. It looks like something is hidden behind it. I found a note that says:

‘Five keys – one opens the door. One key is clear to see, the other four must be found.’

Next to the note, there’s a small inscription: ‘A-B-C-D-E’, and below it, some numbers:

1-3-2-4-5. How do I solve this?