AWK – Joining Multiple Columns from Different Files

I have 4 tsv (tab separated) files that look like this:

file_1:

abc 1
def 2
ghi 3

file_2:

abc 2
ghi 3

file_3:

def 1
ghi 2
jkl 4

file_4:

ghi 3
jkl 4

I want to join those file to get 1 tsv file like this:

dataset file_1 file_2 file_3 file_4
abc     1      2             
def     2      4            
ghi     3      3      2      3
jkl                   4      4

I have try using awk

$ awk '
    BEGIN{OFS=FS="\t"} 
    FNR==1{f = f "\t" FILENAME} 
    NR==FNR{a[$1] = $2} 
    NR!=FNR{a[$1] = a[$1] "\t" $2} 
    END{printf "dataset%s\n", f; for(i in a) print i, a[i]}
  ' file_{1..4}

This command is work, but I got shifted value. Let say, if first and second column have empty value and third and fourth column have value 4 and 4, the output that I got from that command is for first and second column have value 4, but for third and fourth column have empty value.
So I try to join my tsv file separately using awk that I mentioned. First only for file_1 and file_2 to get output_1, then join file_3 and file_4 to get output_2. After that I use $ join output_1 output_2 to merge output_1 and output_2 but I only get value that exist in 4 file. I lost data that only exist in one file.

I'll very appreciate if you can give me an advice.

Thank you