Are there any details on when WP 4 will be released?
Is it true it will be a fundamental re-write using considerable Ajax?
I am really interested in the following issue which is reported as included in 4.0
WordPress 4.0 Release:
For WordPress 4.0 features follow http://make.wordpress.org Planned features at the moment: http://make.wordpress.org/core/2014/05/09/summary-of-5-7-dev-chat-irc-log/
If by AJAX rewrites you mean significant rewriting of the wp-views shortcode system backed by Backbone and Ajax, yes, it is significantly different than 3.9.
From what I've tested, this is the most significant change as it is still a bit buggy and breaking existing code - see for example 29115 (mostly fixed by now), renamed important constants used in TinyMCE modules that could break some plugins (28458, 29130)