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?

your baby is just 4 months old but yes, as Chetna mentioned, you can definitely go for mustard oil. it is one of the best massage oils for anyone, including babies, and as it also has antibacterial properties, it keeps the skin protected along with keeping it soft and nourished. however, if your baby has any dry patch on the skin, mustard oil will cause a burning sensation. so i would suggest first massage your baby using coconut oil for a week or two so that the skin is already nourished. you can then start massaging with mustard oil.