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 ?

Almost all school-going children in Singapore are bilingual as they learn both their Mother Tongue and English. If you're referring to a third language, there's absolutely no problem at all. Children absorb very quickly. You can use this boy as motivation! 10 langauges at the age of 10. http://sg.theasianparent.com/teach-kid-two-languages/