As DrSquishy has commented already, the "weird" lighting is likely a result of the Source engine doing repeated light bounces. Images 2 and 4 are caused by the sun shining in at an angle, so one of the walls is casting shadows on the other wall. Image 3 is caused by the roof casting shadows. The easiest fixes would be to just add some light entities to these areas. Images 1 and 3 could alternatively be fixed by having a more overhead sun angle.