You can also leave luggage at the airport, if that's easier.
As far as getting there, yeah that's pretty much it. Take C1/C10 to Chamartín and then change to C4 to Colmenar Viejo. Note that there are two branches of C4 line and only the Colmenar one passes by Tres Cantos. I imagine you're staying either at the Holiday Inn Express or the Quo which is near the station and attached to the station building respectively. Be sure you are getting on the cercanías train rather than the metro. The tickets are from the red machines and the entrance is at the far north end of the terminal past the metro entrance.
Conversely you can also take the 827 bus directly from T4 and it should take about an hour and drops you right at the train station. In that case you would just pay the driver directly in cash.
For the return trip, you'd have to get either the 5:59 train or 6:24 leaving Tres Cantos to make it on time.
You can consult the schedule here:
http://www.renfe.com/EN/viajeros/cercanias/madrid/
Realistically, if it's possible, a taxi would be loads more convenient though it would be 30-35€ each way.