Actually that's incorrect, not sure where you got the idea that 1:4 is a hard cap for residential

under URA master plan (as of 2021), 1:4 is on the "lowest density" side

Singapore can go all the way up to 2:8 or higher, they do however limit the number of stories (for safety reasons, as their land is not suitable for multiple skyscrapers)



Note that this is for residential use land.

KL's cap of 1:10 and their subsequent implementation of capping the maximum number of storeys was done in mid 2019, so it's a recent development. Before that it was higher. Mega projects with higher plot ratios that were approved earlier had to get special approval.

Also note that these higher ratios are given to commercial land (read: "service apartments"), residential use land in Malaysia still has a much lower cap. Such land is rare and expensive these days, or has been already developed. Which is why you see most high rise projects now are classified as service apartment, not condominium.