Hi there, my son is just over 4 months old and he has started snuggling a thin muslin comforter at bedtime. I know recommendations state not to have anything soft/extra in the cot but is it ok to let him sleep with it if it is breathable and he has good neck strength?

Red Nose recognises the importance of keeping baby’s head & face uncovered in reducing the risk of Sudden Unexpected Death in Infancy (SUDI)
Red Nose also has the following information about soft toys &/or comforters.

Keep soft toys out of the sleeping environment for babies under seven months of age because they may cover the nose and mouth and interfere with breathing.
The risk posed by suffocation by the presence of soft objects in the baby’s sleeping environment outweighs any benefit to the baby from a soft toy.
It is therefore advised not to place soft toys and other soft objects in the cot for babies under seven months of age.
https://rednose.org.au/article/soft-toys-in-the-cot

Is your baby rolling yet?
If he is then it is important to keep his sleep environment clear of soft or padded objects at this stage of learning to roll.
It is also important that he is no longer wrapped/swaddled
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over