Help! Need advice identifying the leads on this transformer

It's not as obvious as it might be, but the label tells you to put line voltage onto pins 1 and 4 whatever voltage you want to run from.

Then for use on 230V, you link pins 2 and 3 together, putting the windings in series without cancelling each other out.

Or for use on 115V, you link pins 1 and 2 together, and pins 3 and 4 together. That way means the 115V windings act in parallel, again without cancelling each other out.

Edit: This arrangement allows the use of a DPDT switch as a line voltage selector in a way that makes it safe to change while powered up, and there are plenty on the market suitably labelled (usually slide switches). Here's how you'd connect it up, it has a rather pleasing symmetry...

I thought I'd provide a picture to illustrate Finbarr's description, as I think that may help:

Just to make things explicit.