4. If in a three-digit number any two adjacent digits differ by no more than 1, then it is called a "smooth number". The number of smooth numbers is $\qquad$ .
Let the three-digit number be $\overline{a b c}$, when $b=0$, $a \in\{1\}, c \in\{0,1\}$;  
when $b=1$, $a \in\{1,2\}, c \in\{0,1,2\}$; when $b=9$, $a \in\{8,9\}, c \in\{8,9\}$; when $b=i(i=2,3, \cdots, 8)$, $a \in\{i-1, i, i+1\}, c \in\{i-1, i, i+1\}$.

Therefore, the number of steady numbers is $2+2 \times 3+2 \times 2+7 \times 3 \times 3=75$.