oracle - to_timestamp encounter the fractional seconds must be between 0 and 999999999 -

enter code herethis code

select to_timestamp('03-apr-13 01.15.31.6754542 pm','dd-mon-rr hh.mi.ss.ff4 am') dual;   

i intended accurate millisecond 4 digits,but error display.and find if digits of .ff equal or greater 7 digits of 6754542, example:

select to_timestamp('03-apr-13 01.15.31.6754542  pm','dd-mon-rr hh.mi.ss.ff8 am') dual; select to_timestamp('03-apr-13 01.15.31.6754542  pm','dd-mon-rr hh.mi.ss.ff9 am') dual; 

and on. disply 03-apr-13 01.15.31.675454200 pm,9 digits... how can accurate millisecond 4 digits,or forever not? please me,thanks.

select    to_char(     to_timestamp('03-apr-13 01.15.31.6754542  pm',                'dd-mon-rr hh.mi.ss.ff am'),   'dd-mon-rr hh.mi.ss.ff4 am') dual;