We just did this yesterday. Sol to T4. It was fine, but allot an hour bc you do need to change lines twice. We took 1 > 6 > 8. It was a lot of walking and up and down stairs, so take that into consideration. We were a family of five and would have taken a taxi, but didn't want to bother trying to find one to fit us at 7am.
Also, do allot more time than usual to actually be at the airport. We checked in at T4, but then had to ride another shuttle to out gate, which tacked on an extra 15 minutes. Don't cut it close.
RS mentions the Aerobus, but I couldn't get clear info on that and I understand it only leaves from Plaza de Cibeles or Atocha, which were further than the 2 minute walk we had to the Sol Metro stop, so didn't seem worth it.