Did you not get any investigation done at that time?
Now that it has already been 4 yrs since, and it still has a dent and is painful so I suggest at least get an X-Ray skull ( AP and Lateral view )
Off course for certain types of skull fractures
you need to get CT scan
brain (plain) done.
Do not worry most of the skull fractures heal on their own . Neurologically intact patients with simple linear fracture and neurologically intact patients with depressed skull fractures are managed conservatively.
As you do not have any other symptoms of convulsion
, altered sensorium and severe headache
so you just need evaluation to find out the cause of indentation of skull and any underlying injury