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So maybe something more like:
awk '
BEGIN { FS = OFS = ","
}
FNR == NR {
car[$1] = $2
next
}
$2 in car {
$4 = car[$2]
}
1' b.csv a.csv
would come closer to what you need???

If you choose not to show us sample data that reflects the contents of your actual files, the likelihood of getting a suggestion that will work for you diminishes greatly. If what you are given as...

You say that the input files are CSVs, but that usually means that a single character is used to separate fields in those files. Instead of that, the files you have shown us have varying numbers of...