Using commerce module for online payment for just 4 product/services? I starting a website which is offering 4 Apartaments to rent. The visitor will be able to make online payments. My question: Should I use the  Drupal Commerce module for this ?

Will I be able to treat each Apartment as a product? I need to include a map and a calendar for each one. Can I accomplish this with the commerce module.

There is no much documentation about this module and it seems complicated....
If all you'd do was display the rooms and manage their bookings, then I would say 
Commerce was not only excessive, but perhaps even entirely wrong.

However, since you mention online payments, I think it could be a good fit, for managing of those. No other contrib module is likely to have as many "out of the box" solutions for payment today, or developed tomorrow.