Hi Mohit,
if you have a downtime the best way is to export the system with SWPM and R3load and import it again while this time using a higher number of data files. This will ensure that the data is evenly distributed over all files.
If you just add files right now all current data remains in the first 4 files while most of the new data will go into the newly added ones. Usually one tries to avoid that as it can impact performance.
Regards,
Beate