I am a number with 6 digits.
None of my digits are repeated.
My 'tens' digit is the sum of its two neighbours.
My 'units' digit is half of my 'hundreds' digit.
My 'thousands' digit is 8.
My 'hundreds' digit is equal to a quarter of my 'thousands' digit.
My first digit is 4.
My 'tens of thousands' digit is greater than its two neighbours.

Who am I?