If you've been paying any attention at all, you're probably wondering why 9.4 isn't out yet. The answer is that we had to change JSONB at the last minute, in a way that breaks compatibility with earlier betas.
In August, a beta-testing user reported that we had an issue with JSONB not compressing well. This was because of the binary structure of key offsets at the beginning of the JSONB value, and the affects were dramatic; in worst cases, JSONB values were 150% larger than comparable JSON values. We spent August through September revising the data structure and Heikki and Tom eventually developed one which gives better compressibility without sacrificing extraction speed.
I did a few benchmarks on the various JSONB types. We're getting a JSONB which is both faster and smaller than competing databases, so it'll be worth the wait.
However, this means that we'll be releasing an 9.4beta3 next week, whose JSONB type will be incompatible with prior betas; you'll have to dump and reload if you were using Beta 1 or Beta 2 and have JSONB data. It also means a delay in final release of 9.4.