No, no, no. Karo syrup should not be given to a baby to make them sleep and skip a feeding. Your baby is only 4.5 months old. Also, at 4.5 months old, why table food already? It isn't recommended a baby start on "baby food" until 6 months of age. This is per their physical and physiological internal development of their organs... their kidneys/digestive system is STILL developing and if they get too much concentrated proteins (such as in eggs at this age), it can overload their kidneys etc., for example.
Karo syrup is SOMETIMES used for constipation. Not as a food.
It is NORMAL for a baby that age to wake up at night... and at 3:30, if he is waking up, that is pretty darn good. You are lucky. At certain ages, they go through "growth spurts" and need to feed more as they get more hungry. If your baby is sleeping through the night at this young age, and only wake up at 3:30a.m., that is darn great. Remember, "sleeping through the night" means, FOR A BABY, sleeping at least 6 hours straight.
Perhaps research proper foods that a baby can have, for each month of age. There are clearly delineated guidelines about this. And consult your Pediatrician. If your main concern is making him sleep through the night without waking up... well, good luck. A baby is a baby, and this is their normal behavior. They will wake up.
To be honest, I am alarmed that he is eating "table food" already, and is 20 lbs. at his age. What has your Pediatrician said about this?
Take care, I don't advise Karo. Ask your Pediatrician.