I have a 4 month old baby who is very active at night and frequently rolls onto her side and rotates around the cot (sometimes ending up upside down). We discontinued swaddling when she started rolling and gave her in a sleep sack. However we are hoping in winter to add a pocket sheet like the bubba blue sleep pouch. I understand the American academy of paediatrics only recommends sleeping bags. Is it safe to use the sheet?

Many parents find that once a child is rolling, that often they do not stay under any bedding - sheet or blanket & often do not use bedding to cover baby.
Red Nose also recommends the use of a safe sleeping bag.
https://rednose.org.au/article/what-is-a-safe-sleeping-bag
If you are concerned about baby being cold during winter, you can use a warmer sleeping bag &/or extra layer of clothing on baby (under sleeping bag.)

Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.
Sleeping your baby in a safe sleeping bag is a great alternative to bedding and can prevent bedclothes from covering your baby’s face.

A good way to check your baby’s temperature is to feel their tummy or back which should feel warm (don’t worry if their hands and feet feel cool, this is normal).

https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep