Hi. My 4 month is not sleeping as well now that it's winter. I am concerned perhaps that the cotton fitted sheet gets cold on his face. Are you able to use a fitted flanellete sheet instead or do these not breathe well enough?

Are you still wrapping your baby?
Is your baby rolling?
Is he in a cot or a bassinet?
How much bedding does he have?

Generally, it is not necessary to use flannelette sheets, as most babies will sleep ok. as long as their core temperature is warm enough.
Most flannelette sheets are predominantly light weight cotton.