Shell Programming and Scripting

Hi All the Helpers!
I have a text file which looks like input.txt.I would request to please suggest me how can I make this file look like output.txt

input.txt

Code:

VOP       111      0 1 2
DEM       111      0   222 333 444 555
                         879 888 987 888 989

VOP       118     0 12 3 6
DEM       118     0 333 555 768 946
                        878 889 987 888 989

output.txt

Code:

VOP       0001      0 1 2
DEM       0001      0   222 333 444 555
                           879 888 987 888 989

VOP 0002 0 12 3 6
DEM 0002 0 333 555 768 946
878 889 987 888 989

So I need that the second column value get replaced by 0001,0002,0003 and goes on with incremantal 0001.

Your help appreciated as always!!
Best Regards

Code:

awk '/VOP/{s=sprintf("%04d",++x);$2=s}
/DEM/{$2=s}1' file

Thanks a lot,will get back if any problem