How to get user_meta value for new user regsitered?

Read de Codex entry for user_register action, it says:

Not all user meta data has been stored in the database when this
action is triggered.

Note that doing:

$user = wp_insert_user( $userdata );
update_user_meta( $user, 'companyId', 350 );

Follow this sequence: insert user -> run user_register action -> rung your update user meta function. So, your custom user meta is not available in user_register action.

So, instead of updating user meta data after wp_insert_user(), you could do it inside the user_register action:

add_action( 'user_register', 'sendMailM' );
function sendMailM( $user_id ) {

    // Note: $_POST data is available here,
    // just in case you need to update user meta based on form input,
    // for example, $_POST['companyId']
    update_user_meta( $user_id, 'companyId', 350 );

    $title  = "Title";
    $from   = "[email protected]";

    global $wpdb, 
        $password;

    $user       = new WP_User( $user_id );
    $user_login = stripslashes( $user->user_login );
    $user_email = stripslashes( $user->user_email );
    // You can now access to previously updated user meta
    // Or get the companyId directly from $_POST input if needed
    $companyCreatedUserP4 = get_user_meta( $user_id, 'companyId', true );

    if( ! empty( $companyCreatedUserP4 ) )
    {
        $current_companyP4  = new WP_User( $companyCreatedUserP4 );
        $companyEmailP4     = $current_companyP4->user_email;
        $messageAdmin       = 'New User ' . $user_login . ' is registered on your site under ' . $current_companyP4->user_firstname . ' company.';
        $messageCompanyP4   = 'A new user with a user name: ' . $user_login . ' was registered under your company.';
    }
    else
    {
        $messageAdmin = 'New User ' . $user_login . ' is registered on your site.'; 
    }

    $message  = "<p>You are now registered . Your user name and password are included in this email. </p>";
    $message .= "<p>" . sprintf( __( 'Username: %s' ), $user_login ) . "\r\n\r\n</p>";
    $message .= "<p>" . sprintf( __( 'Password: %s' ), $passwor d) . "\r\n</p>";

    $headers="From: " . $title . '<' . $from . ">\r\nReply-To: " . $from;
    add_filter( 'wp_mail_content_type', create_function( '', 'return "text/html";' ) );
    wp_mail( get_option( 'admin_email' ), 'New User Registration ', $messageAdmin, $headers );  /*admin*/

    if( ! empty( $companyEmailP4 ) )
        wp_mail( $companyEmailP4, 'New User Registration ', $messageCompanyP4, $headers );      /*user*/
}