It depends also on your hardware and whether you want to invest into new hardware.
example:
SHIII need a graphic card which supports shader up to 1.4
SHIV need a card with shader 2.0 as minimum
There are many threads in forums where customers bought SHIV and ask why they get an error message concerning shader's and SHIV does not run. The reason is, they did not consider the system requirements and still use a graphic card of the GeForce4 or Radeon 9500 generation in their PC.