ActiveSync 4.0 is already available for download. So far, only the English version is available. The download file is just under 6MB.
Note: I have not installed it and could not find any information on requirements so be careful (i.e., backup your data) if you decide to install it!
Update:It looks like the link above is to the developer preview version of ActiveSync 4.0 rather than to the final version. So definitely exercise caution if you decide to install it! Thanks to Jason for the heads up!