4. For any real numbers $x, y$, define the operation $x * y$ as $x * y=a x+b y+$ $c x y$, where $a, b, c$ are constants, and the operations on the right side of the equation are the usual real number addition and multiplication. It is known that $1 * 2=3,2 * 3=4$, and there is a non-zero real number $d$, such that for any real number $x$ we have $x * d=x$, then $d=$ $\qquad$
4. For any real number $x$, we have
$$
\begin{array}{cc} 
& x * d=a x+b d+c d x=x_{1} \\
\therefore & 0 * d=b d=0 . \\
\because & d \neq 0, \quad \therefore \quad b=0 .
\end{array}
$$
(where $d \neq 0$),

Thus, from
$$
\left\{\begin{array}{l}
1 * 2=a+2 b+2 c=3, \\
2 * 3=2 a+3 b+6 c=4 .
\end{array}\right.
$$

we get
$$
\left\{\begin{array}{l}
a+2 c=3 \\
2 a+6 c=4
\end{array}\right.
$$

Therefore, we have
$$
\left\{\begin{array}{l}
a=5 \\
c=-1
\end{array}\right.
$$

Also, from
$$
1 * d=a+b d+c d=1
$$

we get
$$
d=4 \text {. }
$$