A step-by-step walkthrough and tutorial of Alloy4 is http://alloy.mit.edu/alloy4/tutorial4/
and http://alloy.mit.edu/alloy4/tutorial/.

A list of frequently asked questions is here.

The grammar is here.

For existing Alloy 3 users, please refer to the Alloy 4 quick guide for the list of changes, as well as an overview of the new analyzer.

For new users, we are in the process of updating the existing user manual and tutorials for Alloy 4.
In the mean time, please refer to the old Alloy 3 documentation in conjunction with the Alloy 4 quick guide.