UNIX for Dummies Questions & Answers

header1,header2,header3,header4
a,b,c,1
e,f,g,2
h,g,h,3

a,b,c
e,f,g
h,g,h
g,j,g,5
g,h,y,7

header1,header2,header3,header4
a,b,c,1
e,f,g,2
h,g,h,3
g,j,g,5
g,h,y,7

awk -F',' 'NR==FNR{a[$1","$2","$3","$4]=$0;next} $0 in a {$0=a[$0]}1' OFS=, File1 File2

cat file1; tail -n+$(wc -l < file1) file2
header1,header2,header3,header4
a,b,c,1
e,f,g,2
h,g,h,3
g,j,g,5
g,h,y,7

awk -F, NF==4 file1 file2

Might work for these trivial examples:
 
grep ',.*,.*,' file1 file2
 
More serious, assuming file1 only has header:
 
(echo;cat file2)|paste file1 -|sed '
  s/\t.*,.*,.*,/,/
  s/\t$//
 '

awk 'NR==FNR{n=NR-1; print; next} FNR>n' file1 file2