Hi Avadhoot,
Please have look at SAP Note 984229 - F4 modes for input help as of SAP NetWeaver 2004s BI for more information on the different F4 modes.
You might also want to consider implementing BAdI RSR_VARIABLE_F4_RESTRICT_BADI. Please refer to BW: Restricting the value help in the variables screen of a query for an implementation example.
Best regards,
Sander