Assign the node to a user I'm using Drupal 7 and workflow module for Article content type. 
There are two roles author, editor. There can be as many number of authors as how many we want, but there are only four editors. 
Workflow states: Review, Back to Author and Publish.

Worflow offers very good publishing process throught it's rules and actions. Authors create articles, and change the workflow state to Review and save it. Editors have two choices either to publish the content or return it back to the author of the article. Works cool! 

But what I want is to make the authors to choose one of the editors(there 4 of them), that editor is going to be assigned to this article, and only he/she can edit this article while the worflow state changes into Review. Currently any user with Editor role can edit the articles with Review state.
I use workbench, which accomplishes the same thing as workflow.  There is a module in the workbench collection called Workbench access that I use to send content to specific editors in the workflow.  I use it for my school newspaper so that writers moderate content to the editors of their specific section (i.e. news, sports, arts, lifestyle, etc.) then the Editors send the content to me for final approval and publication on the site. Hope this helps!