How can I make Open4 interactively capture all stdout output?

Question

I'm trying to execute a shell command in a Ruby script. The shell command in question (pg_dump) prompts the user for a password. What I would like to do is execute the shell command in a child process and have my parent process interactively enter the password.

I would like for all stdout and stderr to be captured by the parent process so that it is not displayed to user.

Here's some code. All it is supposed to demonstrate is that Open4 is able to capture stdout from the shell command. Unfortunately, it's not doing that.

puts "Executing \"#{pg_dump_command}\""

stdin = ''
stdout = ''
stderr = ''

thread = Open4::background(pg_dump_command, 0 => stdin, 1 => stdout, 2 => stderr)

while(status = thread.status)
  puts '.'

  sleep 1
end

puts "STDOUT: #{stdout}"
puts "STDERR: #{stderr}"

Here's the output. Note that it prints "Password:", despite the fact that Open4 is passed a string for stdout.

Executing "pg_dump -U myuser -Fc mydb > /tmp/db.dump"
.
Password: .
.
(the dots continue for as long as a password is not entered)
STDOUT: 
STDERR: pg_dump: [archiver (db)] connection to database "mydb" failed: FATAL:  password authentication failed for user "myuser"

What am I missing? Thanks for any help.