このようなものが動作するはずです:
# run from your D:\Movies (or whatever) folder
# Go through all subfolders of the folder we're currently in, and find all of the .MP4
# files. For each .MP4 file we find...
ls -Recurse -Filter *.mp4 | %{
# Get the full path to the MP4 file; use it to find the name of the parent folder.
# $_ represents the .MP4 file that we're currently working on.
# Split-Path with the -Parent switch will give us the full path to the parent
# folder. Cast that path to a System.IO.DirectoryInfo object, and get the
# Name property, which is just the name of the folder.
# There are other (maybe better) ways to do this, this is just the way I chose.
$name = ([IO.DirectoryInfo](Split-Path $_.FullName -Parent)).Name
# Tell the user what we're doing...
Write-Host "Renaming $_ to $($name).mp4..."
# Rename the file.
# We have to provide the full path to the file we're renaming, so we use
# $_.FullName to get it. The new name of the file is the same as that of the
# parent folder, which we stored in $name.
# We also remember to add the .MP4 file extension back to the name.
Rename-Item -Path $_.FullName -NewName "$($name).mp4"
}