C 64 63

a,i;main(){for(;i++-4;printf("%d",(a-a/10)%10))scanf("%d",&a);} 

Reads the four numbers in a loop and then processes each by subtracting the first digit from the value and printing the result modulo 10.

C 64 63 56 or 61

If the input can be piped from file

main(a){while(scanf("%d",&a)>0)putchar(48+(a-a/10)%10);} 

If the input should be typed to stdin

i;main(a){for(;i++-4;putchar(48+(a-a/10)%10))scanf("%d",&a);} 

Reads the four numbers in a loop and then processes each by subtracting the first digit from the value and printing the result modulo 10.

Savings thanks to various comments below and also using putchar instead of printf

C 64

a,i;main(){for(;i++-4;scanf("%d",&a),printf("%d",(a-a/10)%10));} 

Reads the four numbers in a loop and then processes each by subtracting the first digit from the value and printing the result modulo 10.

C 64 63

a,i;main(){for(;i++-4;printf("%d",(a-a/10)%10))scanf("%d",&a);} 

Reads the four numbers in a loop and then processes each by subtracting the first digit from the value and printing the result modulo 10.