If you're on separate tickets you need to meet the requirements to enter Schengen regardless or Etihad won't let you board in the first place.

As far as where it is, there is a lift by the bathrooms before passport control on the upper floor of T4S that you can use to descend down to floor zero of the downstairs passport control used for remote stands. From there you can get to the stop for the transfer bus.

https://www.aena.es/en/adolfo-suarez...terminals.html

Important caveats:

• I have never done this nor used or even noticed that lift (though I'll be sure to take a look next time I'm through) I am just getting that from looking at a terminal map.
• I am unsure if Ryanair still requires a stamp landside on printed boarding cards for non-EU citizens. Inferring since you're worried about entering Schengen that you are not an EU citizen. This might be a major stumbling block
• As mentioned, Etihad is fully within their rights to deny you boarding if you don't meat the entry requirements for the final destination of your ticket, which would be Spain in this case.