IME, hotels in places like Singapore and HK are used to flights arriving very early and departing very late, and are generally willing to accomodate, especially for status/regular guests. IC SIN is big on upsells, so I wouldn't expect an 8pm checkout for free. If you're entitled to 4pm checkout, they will likely go to 5pm or 6pm for free (unless the hotel is completely full), but 8pm may be pushing it. If you're not willing to roll the dice on a free extension, just contact them and see what they offer.