Can I use the Ergopouch HugMe zip-up blanket for my four-month-old in a cot? I'm just worried if he rolls in his sleep, and can't roll back if the blanket is too tight. Since transitioning him to arms out sleeping, he keeps waking up overnight. We tried the one arm method for a few weeks, and gradually both arms. It's been 3 weeks and we are so exhausted. We are hoping a secure blanket will help with his sleep as he used to have this in his bassinet. He now kicks it free from the sides in the cot, so we have removed it. I'm hoping a more secure blanket will work, but is this safe if he rolls?

Any bedding used for baby’s sleep should not be so tight that it restricts their ability to move freely.
Is he rolling yet?
How does he go with tummy time when awake?

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.
https://rednose.org.au/article/red-nose-safe-sleeping-recommendations

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over  (please read full article)

Good that you have moved him to his cot, & no longer swaddling/wrapping.
Babies need arms free & the space of a cot to roll well.

Are you wanting to use the above product to try to prevent him from rolling?
Red Nose does not recommend any bedding or sleep positioners that restrain a baby to prevent them from rolling.
Many parent no longer use blankets (or similar) once baby is rolling as they do tend to kick any blankets off.

Let me know your responses so I can reply more specifically.