AWK for loop with two input files - for-loop

Question

I have the following files:
A-111.txt
A-311.txt
B-111.txt
B-311.txt
C-111.txt
C-312.txt
D-112.txt
D-311.txt
I want to merge lines of files with the same basename (same letter before the dash) if there is a match in column 4. I have many files so I want to do it in the loop.
So far I have this:
for f1 in *-1**.txt; do f2="${f1/-1/-3}"; awk -F"\t" 'BEGIN { OFS=FS } NR==FNR { a[$4,$4]=$0 ; next } ($4,$4) in a { print a[$4,$4],$0 }' "$f1" "$f2" > $f1_merged.txt; done
It works for files A and B as intended, but not for files C and D files.
Can someone help me improve the code, please?
EDIT - here's the above code formatted legibly:
for f1 in *-1**.txt; do
f2="${f1/-1/-3}"
awk -F"\t" '
BEGIN {
OFS = FS
}
NR == FNR {
a[$4, $4] = $0
next
}
($4, $4) in a {
print a[$4, $4], $0
}
' "$f1" "$f2" > $f1_merged.txt
done
EDIT - after Ed Morton kindly formatted my code, the error is:
awk: cmd. line:7: fatal: cannot open file 'C-311.txt' for reading (No such file or directory)
awk: cmd. line:7: fatal: cannot open file 'D-312.txt' for reading (No such file or directory)
EDIT-all lines not only the first one should be compared
Input file A-111.txt
ID
Chr
bp
db_SNP
REF
ALT
A-111
1
4367323
rs1490413
G
A
A-111
1
12070292
rs730123
G
A
A-111
22
47836412
rs2040411
G
A
A-111
22
49876931
rs4605480
T
C
Input file A-311.txt
ID
Chr
bp
db_SNP
REF
ALT
A-311
Y
17053771
rs17269816
C
T
A-311
Y
22665262
rs2196155
A
G
A-311
1
4367323
rs1490413
G
A
A-311
1
12070292
rs730123
G
A
Desired output file
ID
Chr
bp
db_SNP
REF
ALT
ID
Chr
bp
db_SNP
REF
ALT
A-111
1
4367323
rs1490413
G
A
A-311
1
4367323
rs1490413
G
A
A-111
1
12070292
rs730123
G
A
A-311
1
12070292
rs730123
G
A

Answer 1

Would you please try the following:
#!/bin/bash
prefix="ref_" # prefix to declare array variable names
declare -A bases # array to count files for the basename
for f in *-[0-9]*.txt; do # loop over the target files
base=${f%%-*} # extract the basename
declare -n ref="$prefix$base" # indirect reference to an array named "$base"
ref+=("$f") # create a list of filenames for the basename
(( bases[$base]++ )) # count the number of files for the basename
done
for base in "${!bases[#]}"; do # loop over the basenames
if (( ${bases[$base]} == 2 )); then # check if the number of files are two
declare -n ref="$prefix$base" # indirect reference
awk -F'\t' -v OFS='\t' '
NR==FNR { # read 1st file
f0[$4] = $0 # store the record keyed by $4
next
}
$4 in f0 { # read 2nd file and check if f0[$f4] is defined
print f0[$4], $0 # if match, merge the records and print
}' "${ref[0]}" "${ref[1]}" > "${base}_merged.txt"
fi
done
First extract the basenames such as "A", "B", .. then create a list
of associated filenames. For instance, the array "A" will be assigned to
('A-111.txt' 'A-311.txt'). At the same time, the array bases counts
the files for each basename.
Then loop over the basenames, make sure the number of associated files
are two, compare the 4th columns of the files. If they match, concatenate
the files to generate a new file.
The awk script searches the 4th field across the lines; if match, paste the lines of the two files.