There are 241 reviews for the Cosmopolitan here:
http://www.booking.co.../cosmopolitan.en.html
(Click 'Guest reviews').
They seem to be favourable.
That hotel offers 'executive family quad rooms':
http://www.cosmopolit...utive_familyquad.html
Using that first link above, the price offered for a 4 night stay in June is £462.79, including breakfast. (That's equivalent to £28.92 per person per night).
The Trip Advisor reviews for the Langham Place hotel are favourable, and refer to the availability of quad rooms:
http://www.tripadviso...Hong_Kong_Region.html
I've not checked the prices for the Langham Place. Try their website here
http://hongkong.langhamplacehotels.com/en/
or google to see if there are any booking sites offering discounts.
Chris