If I feel that I can make 3-4% return on my investments through index funds (i.e STI ETF), should I pay my home loan using my cpf oa or using cash instead? ?

Personally, I will advise to pay a portion of it with cash, as CPF is a really good financial instrument for retirement.
Giving 3.5% for your first 20k in OA and 2.5% subsequently.

Taking money out of your CPF, means you will lose the 2.5 or 3.5% of interests and on top of that you are also paying your loan interests of 2.6% (assuming one took a load from HDB) so thats is like a -5%.

But not everyone can afford to pay with cash and also some may earn more than enough for CPF hence its good for them to split as they have surplus from their CPF. (Everyone has a different number so its good to do your maths before deciding on the % split)

Compare apple to apple.

HDB loan is leverage and depends on the type you buy you get capital apprecation.​​​
investments through index funds (i.e STI ETF) is only using your money call cash or CPF.