what are some good massage oils that i can use for my 4 month old? how can i be sure they wont harm the skin?

You can use coconut oil. I used to massage my kids with coconut oil throughout. I used house olive oil in winters. Olive oil is thick in consistency and provides moisture to the skin for longer time. You can also use almond oil as well. Buy the oils from a good company and read the ingredients well. And do not rub too hard as that is the only reason that your baby may get rashes.