What is USON package? Is it the same with UTDFN
It looks like SON and DFN refer to the same package. JEDEC documents like this one talk about "SON/QFN", but on the whole JEDEC site I couldn't find one reference to "DFN".  

On the other hand, among its dozens of packages Linear Technology lists DFN and QFN, but there's no trace of SON.

JEDEC describes the USON as:  


  The Plastic Ultra-Thin Small Outline No-lead Package(USON) is a rectangular
  semiconductor package with metal terminals along two sides of the bottom of
  the package. The terminals are either flush with bottom or protruding slightly
  below bottom of package. The main body of the component is generally a
  molded plastic.


So a SON package, having its terminals on two sides, is much like a DFN. So 4 pins on 2 sides would be SON/DFN. A QFN-12 may have 4\$\times\$3 terminals, but other configurations like 2 \$\times\$ (5 + 1) are also possible.  


  

This may cause some confusion if we look at the following package:  



which appears to be a DFN-16 and not a QFN-18.