My baby is only 4 weeks old but is wriggling to the side of the mesh bassinet at night. She is wearing a LTD sack and an Ergo tuck sheet. Should we try transition to an arms out sack? She is still too young to roll, but I’m worried that she’s continually pressing her face against the mesh.

Most babies will be rolling by 4 - 5 months, however some babies roll for various reasons before that.
Some babies do not respond well to wrapping & like their arms free.

Often these babies like their arms free &/or to touch their face or suck fingers & will wriggle to get free.
In their wriggling they may get onto their side (some even their tummy)

It is worth trying her with arms out of the swaddle suit or use a muslin wrap under the arms wrapping her body only, or you could cease wrapping altogether.

If the rolling continues, try sleeping her in her cot instead of bassinet for the extra amount of room.
Still continue to place her on her back for sleep.

How does she go with tummy time when awake?