S$4000 is probably just the papers alone
CAAS requires you to be enrolled into a CAAS approved school in order to sit for the ATPL exams.
you can enrol into STAA's CPL/IR course but I don't think you're allowed to just go for ground school, it comes as a package. You might have penalties to pay if you choose to "drop out" after ground school is completed
during ground school phase, you have 3 payment milestones, each about S$15,600 if I'm not wrong.
total is roughly S$47,000. Note that this amount is NOT the actual cost of ground school. The payment milestones is spread out and arranged as a whole CPL course package.
the CPL course is roughly S$140k in total