what about this?

> cat -n big_file4 | awk '{printf "%1s %-100s \n", substr($1,length($1),1), $0}' | cut -c1,10- | grep "^[147]" | cut -c2- >filea
> cat -n big_file4 | awk '{printf "%1s %-100s \n", substr($1,length($1),1), $0}' | cut -c1,10- | grep "^[258]" | cut -c2- >fileb
> cat -n big_file4 | awk '{printf "%1s %-100s \n", substr($1,length($1),1), $0}' | cut -c1,10- | grep "^[369]" | cut -c2- >filec
> cat -n big_file4 | awk '{printf "%1s %-100s \n", substr($1,length($1),1), $0}' | cut -c1,10- | grep "^[0]" | cut -c2- >filed

> cat big_file4
a stuff to 1 file
b stuff to 2 file
c stuff to 3 file
a stuff to 1 file
b stuff to 2 file
c stuff to 3 file
a stuff to 1 file
b stuff to 2 file
c stuff to 3 file
d stuff to 4 file
a stuff to 1 file
b stuff to 2 file
c stuff to 3 file
a stuff to 1 file
b stuff to 2 file
c stuff to 3 file
a stuff to 1 file
b stuff to 2 file
c stuff to 3 file
d stuff to 4 file

> cat filea
a stuff to 1 file                                                                             
a stuff to 1 file                                                                             
a stuff to 1 file                                                                             
a stuff to 1 file                                                                             
a stuff to 1 file                                                                             
a stuff to 1 file                                                                             
> cat fileb
b stuff to 2 file                                                                             
b stuff to 2 file                                                                             
b stuff to 2 file                                                                             
b stuff to 2 file                                                                             
b stuff to 2 file                                                                             
b stuff to 2 file                                                                             
> cat filec
c stuff to 3 file                                                                             
c stuff to 3 file                                                                             
c stuff to 3 file                                                                             
c stuff to 3 file                                                                             
c stuff to 3 file                                                                             
c stuff to 3 file                                                                             
> cat filed
d stuff to 4 file                                                                             
d stuff to 4 file                                                                             
>