awk sub ++count every 4 matches unlike every 1 match - unix

Question

Let's say I have the following 1.txt file below:
one file.txt
two file.txt
three file.txt
four file.txt
five file.txt
sixt file.txt
seven file.txt
eight file.txt
nine file.txt
I usually use the following command below to sequentially rename the files listed at 1.txt:
awk '/\.txt/{sub(".txt",++count"&")} 1' 1.txt > 2.txt
The output is 2.txt:
one file1.txt
two file2.txt
three file3.txt
four file4.txt
five file5.txt
sixt file6.txt
seven file7.txt
eight file8.txt
nine file9.txt
But I would like to rename only every 4 matches when the pattern is .txt.
to clarify, a pseudocode would be something like:
awk '/\.txt/{sub(".txt",++count"&")} 1 | <change every 4 matches> ' 1.txt > 3.txt
such that 3.txt is as below:
one file.txt
two file.txt
three file.txt
four file1.txt <-here
five file.txt
sixt file.txt
seven file.txt
eight file2.txt <- here
nine file.txt
I have been looking for both the web and in my learning and I do not remember something like that and I am having difficulty starting something to achieve this result.
Note: Maybe I just need to continue the command below:
awk -v n=0 '/\.txt/{if (n++==4) sub(".txt",++count"&")} 1'

Answer 1

Adding 1 more awk variant here, based on your shown samples only. Simple explanation would be, check if line is NOT NULL AND count variable value is 4, then substitute .txt with count1(increasing with 1 each time) with .txt itself and print the line.
awk 'NF && ++count==4{sub(/\.txt/,++count1"&");count=0} 1' 1.txt > 2.txt

Answer 2

You are almost there. Would you please try:
awk '/\.txt/ {if (++n%4==0) sub(".txt",++count"&")} 1' 1.txt > 2.txt
The condition ++n%4==0 meets every four valid lines.

Answer 3

Another option could be passing n=4 to use it for both the modulo and the division.
For the string you could pass s=".txt" for index() to check if it present and for sub to use in the replacement.
awk -v str=".txt" -v nr=4 'index($0,str){if(!(++i%nr)) sub(str,i/nr"&")}1' 1.txt > 2.txt