#!/bin/Perl
use warnings;

if (@ARGV < 4) {
    print ("you need to provide 4 arguments\n\t4th argument is file to
+ parse");
    exit;
    
}

$fh = pop(@ARGV);

openfile(@ARGV);


sub openfile {
    
    open FILE1,">$_[0]" || die "could not open $_[0]";
    open FILE2,">$_[1]" || die "could not open $_[1]";
    open FILE3,">$_[2]" || die "could not open $_[2]";
    open FILE4,"<$fh" || die "could not open $fh";
    
        while (<FILE4>) {
    
print $_;    
}
    
    

}
[download]

this makes no sense why the FILE4 file handle wont be printed i opened it for reading

from open,

If MODE is ">", the file is opened for output, with existing files first being truncated ("clobbered") and nonexisting files newly created. If MODE is ">>", the file is opened for appending, again being created if necessary.
So, your files $_[0] to $_[3], is been "clobbered", by the ">" mode, while the last one should print out.

If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

Another large bug in your code is that due to operator precedence, your dies won't work.

Basically, open FILE1,">$_[0]" || die "could not open $_[0]" is being interpreted as open FILE1,(">$_[0]" || die "could not open $_[0]")

$ perl -MO=Deparse -e 'open INP, "<", "hello" || die $!'              
+                               

open INP, '<', 'hello';
-e syntax OK



$ perl -MO=Deparse -e 'open(INP, "<", "hello") || die $!'             
+                                         
die $! unless open INP, '<', 'hello';
-e syntax OK



$ perl -MO=Deparse -e 'open INP, "<", "hello" or die "could not open: 
+$!"'                                                      
die $! unless open INP, '<', 'hello';
-e syntax OK
[download]

You either need to use the lower-precedence or operator or call the open() function with parentheses.