  The Diophantine equation $A^4+hB^4=C^4+hD^4$, where $h$ is a fixed arbitrary
positive integer, has been investigated by some authors. Currently, by computer
search, the integer solutions of this equation are known for all positive
integer values of $h \le 5000$ and $A, B, C, D \le 100000$, except for some
numbers, while a solution of this Diophantine equation is not known for
arbitrary positive integer values of $h$. Gerardin and Piezas found solutions
of this equation when $h$ is given by polynomials of degrees $5$ and $2$
respectively. Also Choudhry presented some new solutions of this equation when
$h$ is given by polynomials of degrees $2$, $3$, and $4$.
  In this paper, by using the elliptic curves theory, we study this Diophantine
equation, where $h$ is a fixed arbitrary rational number. We work out some
solutions of the Diophantine equation for certain values of $h$, in particular
for the values which has not already been found a solution in the range where
$A, B, C, D \le 100000$ by computer search. Also we present some new parametric
solutions for the Diophantine equation when $h$ is given by polynomials of
degrees $3$, $4$. Finally We present two conjectures such that if one of them
is correct, then we may solve the above Diophantine equation for arbitrary
rational number $h$.
