Non-Decreasing Digits

Letting $i$ correspond to the first digit, $j$ to the second, $k$ to the third and $l$ to the fourth, the number of numbers is $$\sum_{l=0}^9 \sum_{k=0}^l \sum_{j=0}^k \sum_{i=0}^j 1$$ $$= \sum_{l=0}^9 \sum_{k=0}^l \sum_{j=0}^k \binom{j+1}{1}$$ $$= \sum_{l=0}^9 \sum_{k=0}^l \binom{k+2}{2}$$ $$= \sum_{l=0}^9 \binom{l+3}{3}$$ $$= \binom{9+4}{4} = 715$$

Let the four digits be $d_1,d_2,d_3$, and $d_4$ from left to right. Let $a_1=d_1,a_2=d_2-d_1,a_3=d_3-d_2$, $a_4=d_4-d_3$, and $a_5=9-d_4$. Note that each non-decreasing four-digit number corresponds to a unique $5$-tuple $\langle a_1,a_2,a_3,a_4,a_5\rangle$ of non-negative integers such that $a_1+a_2+a_3+a_4+a_5=9$ and vice versa: given such a $5$-tuple, we can recover the number because $d_1=a_1,d_2=a_1+a_2$, $d_3=a_1+a_2+a_3$, and $d_4=a_1+a_2+a_3+a_4$. (The fifth number, $a_5$, merely serves to ensure that the total is a known number.)

The problem of counting non-negative integer solutions is a standard stars-and-bars problem. The Wikipedia article gives a decent explanation of the reasoning involved; the answer is $$\binom{9+5-1}{5-1}=\binom{13}4=715\;.$$

There are 10!/(4!*6!) = 210 to get 4 different numbers

There are 3* [10!/(3!*7!)] =3*120 =360 to get 3 differnt numbers with 1 repeating

There are 10!/(2!*8!) = 45 to get 2 differnt numbers with each repeating

There are 2* [10!/(2!*8!) ] = 2*45 =90 to get 2 different numbers with 1 repeating 3 times

There are 10 ways to get 1 number repeating 4 times

210 +360 +45 +90 +10 = 715