Following my earlier questions/comments, I have been looking through this thread in detail and am certainly tempted by the Sony RX10.
I am slightly concerned about the size and weight, as in the past when out and about with the family (casual outings, rather than specific photography trips), I have left my gear at home due to the inconvenience of a bulky bag whilst dealing with kids and all the stuff that needs carting around for a day out with them. I wonder therefore if a more compact camera would suit those occasions. But then I look at the versatility of the RX10 and that lovely sharp lens and think how that would suit so many of the things I like to shoot! I don't really want to have two cameras, but maybe that is the answer - small and light for family outings and the larger, more versatile RX10 for other times.
If I do go for the RX10 I would probably go for the III based on cost/budget. The IV feels a bit too expensive for me at the moment. I've read that the improved AF on the IV makes it much better at tracking. Linked to the previous couple of posts, how have those using the III found the AF performance for moving targets (birds in flight, etc)?