4. If in a three-digit number any two adjacent digits differ by no more than 1, then it is called a "smooth number". Then, the number of smooth numbers is $\qquad$ .
4. 75 .

Consider the steady number $\overline{a b c}$.
If $b=0$, then $a=1, c \in\{0,1\}$, there are 2 steady numbers.

If $b=1$, then $a \in\{1,2\}, c \in\{0,1,2\}$, there are $2 \times 3=6$ steady numbers.

If $2 \leqslant b \leqslant 8$, then $a 、 c \in\{b-1, b, b+1\}$, there are $7 \times 3 \times 3=63$ steady numbers.

If $b=9$, then $a 、 c \in\{8,9\}$, there are $2 \times 2=4$ steady numbers.
In summary, the number of steady numbers is $2+6+63+4=75$.