How to start a new column after every nth row?

Question

I have a file (comma separated) on a Linux system with 3 columns. I want to start new column after every 4th row.

Input:

col1,col2,col3 
1,disease1,high
1,disease2,low 
1,disease3,high
col1,col2,col3 
2,disease1,low 
2,disease2,low 
2,disease3,high
col1,col2,col3
3,disease1,low
3,disease2,low
3,disease3,low

Expected output:

col1,col2,col3,col1,col2,col3,col1,col2,col3
1,disease1,high,2,disease1,low,3,disease1,low
1,disease2,low,2,disease2,low,3,disease2,low
1,disease3,high,2,disease3,high,disease3,low

i.e. I want exactly 4 lines of output, each line is the result of joining every fourth line of the input with a comma.

Answer 1

With awk:

awk '{a[NR%4] = a[NR%4] (NR<=4 ? "" : ",") $0}
     END{for (i = 1; i <= 4; i++) print a[i%4]}' < input.txt

Answer 2

Try pasteing four lines into one, reading those into four variables, appending each to output lines:

paste -s -d"   \n" file | 
{ while read A B C D 
    do  L1="$L1$DL$A"
        L2="$L2$DL$B"
        L3="$L3$DL$C"
        L4="$L4$DL$D"
        DL=, 
    done 
  printf "%s\n" "$L1" "$L2" "$L3" "$L4"  
}
col1,col2,col3,col1,col2,col3,
1,disease1,high,2,disease1,low,
1,disease2,low,2,disease2,low,
1,disease3,high,2,disease3,high,

EDIT: or, a bit simpler, no paste needed:

while read A && read B && read C && read D     
  do    L1="$L1$DL$A"
        L2="$L2$DL$B"
        L3="$L3$DL$C"
        L4="$L4$DL$D"
        DL=,     
  done < file 
printf "%s\n" "$L1" "$L2" "$L3" "$L4"