Sgd,
Our Renfe ticket says "Combinado Cercanias". Can we use this for our
travel from Atocha to Airport the next day.
The airport website says " For passengrers with AVE tickets
(high-speed train) connecting with a flight, this service is free."
Not sure if it applies to us.
Is Atocha station or terminal T4 complicated to navigate
No, you cannot use the Combinado Cercanias (Cercanias ticket combined with the AVE ticket) the following day.
Cercanias Combinado can only be used 4 hours prior to the (outbound) AVE departure time & 4 hours after the arrival of the AVE train at your destination.
Puerta de Atocha station is BIG, but all you have to do is buy your Cercanias ticket at the Cercanias ticket counter, then follow the signs & go downthe escalators to the subterranean level & take the C-1 line to the airport.
Here is a Map of Atocha's Station, Cercanias train platforms are on level -1 ( Planta -1 on map) & designated by a red circle with a backward white "C" icon.
EDIT to add... If you will be traveling to the airport the day of your flight, I'd suggest buying your Cercanias ticket(s) the day prior to departure.
BTW, you don't need to go to Atocha to take the Cercanias train. If your hotel is close to one of the stations along the Cercanias C-1 route, then you can take it at that particular station.
Here is a diagram showing the Cercanias network. Cercanias C-1 ( light blue) runs from Príncipe Pío to Aeropuerto T4 ( Airport Terminal 4): http://www.renfe.com/viajeros/cercanias/planos/madrid.html
Hope this helps!
Edit to add:
Please note this important information: Keep your Cercanias train ticket because you need it to exit the station.
Here is another useful website for you: http://www.madrid-traveller.com/transport-from-madrid-to-barajas-airport/
BTW, the hotel I recommended (on my first reply) above, serves a delicious & hearty buffet breakfast for an added fee & it's certainly worth it ;-)