Can I use Singapore Apple Watch 4 + Cellular in Malaysia?

Apple’s current list of supporting networks (Apple Watch - Carriers - Apple) does not include any Malaysian networks so we can’t be certain. However, If or when a Malaysian network offers support the Singapore purchased hardware should be compatible because both countries use the same cellular frequency bands (unlike USA). Whether the new network would accept a foreign purchased watch is impossible to answer, but there is no technical reason why it should not work. Software will not be a problem, all Apple Watch software is worldwide. Warranty is also impossible to answer, I can only refer you to https://www.apple.com/legal/warranty/ and let you make your own judgement with the warning that you are speculating into the future.

On a more positive note, a cellular watch will work perfectly well without the cellular functions active. It will fall back to behave exactly like the GPS model making all its communication through the paired iPhone, so you could continue use the watch in Malaysia while you wait and hope one of the networks will launch support.