Clarification about Market Structure Oligopoly or Monopoly
This market is an oligopoly that is subject to government regulation. 

It cannot be a monopoly because there is more than one firm. The presence of the regulating government body is a "red herring", it distracts from the main point- there are multiple firms. 

It does not appear to be competitive because:


Four is subjectively few firms
Implicitly, regulation would be entirely pointless in a competitive market.
One firm has managed to differentiate themselves by adding air conditioners. Firms in perfect competition cannot be differentiated from one another.