Does learning too many languages affect the learning ability of child in anyway especially Mandarin since it's quite hard to learn. I am not sure if my 4 yr old should learn Mandarin in school. Will it be too hectic for him to learn a difficult language along with all the homework ?

It is always good for kids to learn more new languages. For young kids they learn very fast in the environment. In Singapore, learning chinese in School is based on immersion and I am sure your kid will pick up very fast. In addition, your kid will be able to communicate to people in Singapore in Mandarin since most of the residents here are Chinese Speaking.