Welcome to SC! I had ver I, III and now am using ver 4. III and 4 were used from Keh with their 10% sale. The low light af speed might be better on IV though I have not tested lat the same time. There were months between them. Ver V has a pdf af but I have not tried. I saw sales in the amazon for high 800's when they had one day sale. The most useful upgrade is the min shutter for me. IQ might be similar. Three layers is supposedly some of the panning e-shutter jello issues on v but I have not seen that issue on the other versions for still.