Hi Ravichand,
If a search help is defined for a field and if you want that values to be displayed for a field on webui when the user clicks on f4, so that he can select a value from that possible values displayed in the search help.
Best example is country field, we can use the standard search help CRM_UIU_BP_REGION.
You can do it using the GET_V_XXX method of the attribute.
In dropdown we generally display some selected values which we append to a table and pass to a method.
Example is Status field we display OPEN, CLOSED, REJECTED.. etc.
You can do it using GET_P_XXX, GET_V_XXX methods of the attribute.
It depends on the requirement whether to use F4 or dropdown.
Follow this links this will help you.
http://www.sapcrmwebui.com/2013/05/38-providing-dropdown-list.html
http://www.sapcrmwebui.com/2013/05/39providing-f4-help.html
Thanks and Regards,
DP.