Here's how I understand it: even though there are three IB/OW lounges in the T4/T4S realm, the way it is set up, you don't really have a chance to use more than one.
Sala Velásquez: this is in T4-Satellite and it really set up for people already cleared for departure on non-Schengen/International flights. Not sure you'd be able to access it on a Schengen-Schengen connection (probably) transiting entirely in T4.
Sala Dalí: this is in T4 and would be the natural one for you to use for Schengen-Schengen connections.
Sala "?": there is one other one in T4, the name escapes me, but it is set up for use primarily by folks using the Puente Aereo air shuttle flights between Madrid and Barcelona. It is on a different level in the terminal than Sala Dalí. Again whether or not you'd be able to access it even if you wanted to, I'm not sure since you wouldn't have a Puente Aereo boarding pass.
Don't despair, Sala Dalí is overall pretty great with nice open-to-the-ceiling atmosphere, nice showers, tarmac views, good food during meal times, nice wine, etc.