It's called 'Hoot', not whack.
For a $20 budget for a single number, most common play is Zhup Dua Zhup Sway, $10 Big, $10 Small.
Rationale is to cater for proportion loss/gain for Big Vs Small (approx 2:3 or 1:2) for Top 3 prizes to give a higher overall winning value.
In the event the number heng heng hits a top 3 prize, the total winnings would be higher - $50,000 vs $40,000 if you were to just hoot $20 big. And if hit consolation and below, the difference won't be much to compare if you play $20 Big anyways.
The higher the budget, the more the bet should be split into Big/Small as the difference will be significant.
But if you only buy below $5 for a single no., then forget it, can always just hoot Big will do, lol.
With this in mind, it is never advisable to just buy Small for a given no. Small must always pair up with a Big split in order to maximize the winnings.
Ref:
https://online.singaporepools.com/en/lottery/4d-prize-structure