What are these four little things between the horizontal and vertical stabilizer on the B737?
The objects are vortex generators. In general, vortex generators serve to introduce turbulence which helps to prevent flow separation. More information here:


  This was to reduce elevator and elevator tab vibration during flight to increase their hinge bearing service life.


As mentioned on the site, you can see these on any 737 version from the original -100/-200 up through the NG. Better image from same site: