I was recently given a 4.5tog Marino wool sleeping bag for my 4 month old baby. Is this too hot for my baby and is there a maximum tog rating to use with a baby? My house usually gets down to 15-18C overnight.

It can be difficult to work out what may be the most appropriate clothing/bedding for baby when sleeping.
Red Nose recommends dressing baby according to the room temperature, rather than trying to modify the room temperature with heating or cooling.

I also wonder whether a 4.5 TOG sleep bag would be too warm for your baby? 15 - 18 deg is not significantly cold for winter.
What State are you from?
Its hard to follow the TOG ratings exactly as, like adults, babies needs for warmth will vary.

It is not possible to give you specific guidance for dressing your baby, as every baby is different.
Red Nose has the following general information.

“Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.
Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
Remove hats, bonnets, beanies and hooded clothing from baby’s head as soon as baby is indoors.”  -
Always sleep baby with head & face uncovered.
“A good way to check baby’s temperature is to feel baby’s back or tummy (don’t worry if baby’s hands and feet feel cool - this is normal)...................
Consider how many layers that you as the baby’s carer are wearing comfortably.”
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep
“As long as baby is put down to sleep on their back, and that baby is dressed appropriately for the room temperature – not overdressed or under dressed – with their head and face uncovered, you can feel reassured that you are protecting baby from overheating.
We don’t believe that it’s necessary to use a room temperature monitor, or to leave the heating or cooling on all night, as long as baby is dressed appropriately for the temperature of the room – not too hot, not too cold.”
https://rednose.org.au/article/how-to-dress-baby-for-sleep