Can you use flannette sheets on a cot? I have a 4 month old.

Red Nose recommends that you use cotton or muslin for bedding and light weight blankets for baby.
If you are using flannelette (made from cotton) it is still important to remember not to overdress baby, which may lead to overheating.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep
https://rednose.org.au/article/how-much-bedding-does-baby-need
Ensure there are no small “balls” of fabric that baby might be at risk of inhaling.