I believe V4s and Entamas are very similar, though if memory serves the V4 is basically an updated Entama - five games instead of three, an actual shop, etc. They have slightly different characters as well. Design-wise they are almost identical. I believe the Entama is only available in Japanese.
Personally my favourite is the V4, but the V4.5 is very similar to that (the only differences are characters and games available). In my opinion the games on the V4.5 are slightly easier to master, especially games tailored to specific skill points. There's a fairly complicated skill system but I won't go into it otherwise you'll die of boredom halfway through my post. The V4/.5 operates using a mail-based system. You get a fortune every fifteen minutes (if I remember correctly), so that means a beep and check your mail! The fortune can "predict" things like good or bad health, money, etc. These affect gameplay to some degree - a good fortune with money will mean you earn more from games for a while. There is also a schooling/job system.
The V3 is a lot simpler. Instead of the postal system you get an option to turn out the lights (your tama will beep for you when it falls asleep to turn out the lights. The V4/.5 does this automatically) and there is no skill points system. On the V4/.5, the adult character you get is somewhat based on care but is almost entirely based on how many skill points it has in a certain area. On the V3, it is determined entirely by care and I think odd/even generations factor in.
I've never had a V1 so I can't comment on that.