Hello I have a cot that has high sides and I find it difficult to pick up and put down my 4 month old in it as I have a back injury. Would it be safe to add a second, well fitting mattress on top of the existing one to reduce the distance I have to bend down? Thanks for any advice you can offer.

Red Nose recommends the use of a firm & well-fitting mattress for baby’s sleep environment.
Adding an extra mattress may compromise the firmness of the mattress.
Only use a single, firm, mattress that fits snugly (within 20 mm of sides and ends) into the cot. The mattress must be flat (not tilted or elevated).
https://rednose.org.au/article/what-is-a-safe-cot

This is particularly important as your baby will soon be rolling &/or reaching & pulling up on the sides of the cot…...... if not already.

Product Safe Australia (ACCC) develops & monitor safety standards for cots.
These mandatory standards include the height of the sides of the cot to reduce the risk of baby falling from the cot.
Only use the mattress specifically designed for the cot, and never place an extra mattress in the cot.

https://www.productsafety.gov.au/products/babies-kids/kids-furniture/household-cots

With the mattress base set in the lower position, the cot sides or end need to be at least 500mm higher than the mattress
https://www.productsafety.gov.au/system/files/Safety alert — Cots.pdf

Does your cot have 2 mattress levels?........... do not use upper level if you are concerned baby may be able to pull up on side of the cot when mattress in upper position.
Some parents use a drop side cot when there are difficulties with lifting baby.
(read specific instructions in relation to drop side cots.)
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/household-cots