Cannot Add a checked=checked Attribute to Input tag

Cannot Add a checked=checked Attribute to Input tag

Very simply, I want this function to attach the attribute 
checked=checked to the input

<input type='checkbox' id='ckbox4' name='please select4' value='1234' '/>

The following code does not do this 

$("#ckbox4").attr('checked','checked');

or    $("#ckbox4").attr('checked',true);

or $("#ckbox4").checked='checked' / true

Any help or ideas is greatly appreciated.  Other ways of doing this 
would be great as well.  I just want to add this upon form submission.