Start-SPAssignment -Global
$SiteUrl = "your site url"
$NewSystemMasterPageUrl = "/CorporateCommunications/_catalogs/masterpage/v4.master"
$site = Get-SPSite $SiteUrl
$topWeb = Get-SPWeb $site.Url

foreach ($web in $topWeb.Webs)
{
    #Write-host $web.Title $web.MasterUrl
    $web.MasterUrl = $NewSystemMasterPageUrl;
    $web.AllProperties["__InheritsMasterUrl"] = "False"
    $web.Update();
}

Stop-SPAssignment -Global

Note: Start-SPAssignment here is used for the varialbes disposal purpose