There are often minor updates to Saxperience, but v4 is a complete (100%) re-write. Making significant changes while maintaining minimal downtime takes a lot of preparation, planning and testing.
On the journey so far we've upgraded the server a couple of times and even recoded the entire site (from Win/ASP/MSSQL to Linux/PHP/MySQL) with downtime often measured in minutes, and all the while no posts, topics, threads, features, galleries or anything else has been lost.
v4 is important to Saxperience and it remains an active development project, and the upgrade will happen when it's ready. Sorry - not giving anything away just yet