how to put validation in sharepoint list item for dropdown value that user cannot jump from first to third value

hi , 

i am looking for validation in SharePoint 2010 list  , 

suppose we are having on list in which we have given one drop down value 1234 .

1- when user raise an new item he should select 1 if he select 2 or 3 or 4 it pop should come that its not valid something like 

2- while editing he should select 2 means he can not skip any ordered .. 

please provide any solution for this .  

Abhishek Tiwari