Error: Error occurred in deployment step 'Retract Solution': Cannot start service SPUserCodeV4 on computer.

Solution: 

In the central administration, goto System Settings section, click Manage services on server. Make sure to select the server on which you want to enable enable sand box solution. Now, search for Microsoft SharePoint Foundation Sandboxed Code Service and click 'start'. Here you go, deploy your solution again and you are ready to go.

Reference: http://technet.microsoft.com/en-us/library/ff535775.aspx