Renfe calls it a lanzadera on their website but it's still a train (on their Twitter account they specify they are trenes lanzadera, shuttle trains). I can confirm that on Sunday there were trains every 20 min from T4 to Chamartín (I was on one of them). It took me 70 min to get from my gate to the AVE departure hall in Atocha, which, considering that I arrived at T4S, it's not significantly more than the usual time.

https://x.com/CercaniasMadrid/status/1806292330183360644