Create a subtheme of Omega 4.x in Drupal 7 using drush

Download and install Omega 4 theme as you would for any Drupal Theme or use drush cmd in your website folder (Don't install omega tools)

drush dl omega

drush en omega

Check to see if you have a version of drush with omega helper commands

drush help --filter=omega

All commands in omega: (omega)
 omega-export (oexp)   Exports the theme settings of a given theme from the    
                       database to the .info file.                             
 omega-guard (ogrd)    Runs guard for the given theme including Compass and    
                       LiveReload by default.                                  
 omega-revert (orev)   Reverts the theme settings of a given theme by deleting 
                       them from the database.                                 
 omega-subtheme        Creates a Omega subtheme.                               
 (osub)                                                                        
 omega-wizard (owiz)   Guides you through a wizard for generating a subtheme.

Create a subtheme using drush and follow the prompts

drush omega-wizard

Change directory to your new subtheme and run bundle install to get all gem file dependenices 

bundel install

Run drush omega guard to automatically watch your sass folder for changes and apply to css

drush omega-guard