4. Let $a \triangle b$ and $a \nabla b$ represent the minimum and maximum of the two numbers $a$ and $b$, respectively. For example, $3 \triangle 4=3, 3 \nabla 4=4$. For different natural numbers $x$, the number of possible values for $6 \triangle[4 \nabla(x \triangle 5)]$ is $\qquad$.
【Analysis】First, classify $x$ and derive 2 results respectively, and solve accordingly.
【Solution】Solution: When $x \geqslant 5$, $x \triangle 5=5$, $4 \nabla(x \triangle 5)=5$, $6 \triangle[4 \nabla(x \triangle 5)]=5$; when $x<4$, $x \triangle 5=x$, $4 \nabla(x \triangle 5)=4$, $6 \triangle[4 \nabla(x \triangle 5)]=4$. Therefore, the answer is: 2.