This took somewhat longer than planned. Due to the long lapse between JAGS 3.4.0 and JAGS 4.0.0 there will be quite a long beta period. I shall explain some of the important changes in a series of follow-up posts.
I cannot currently upload the files to Sourceforge. They are still recovering from a major outage from a couple of weeks ago. For now, you can download the source tar balls for JAGS-4.0.0 beta and the R package rjags_4-1 from my Google Drive. (Click the icon and then the download symbol that appears in the black bar at the top of the screen).