Our four week old baby manages to wriggle to the edge of her bassinet that is closest to us every time we put her down. She’s been doing this since she was a week old, despite us always lying her in the middle of her bassinet on her back. She usually sleeps with her forehead against the side of the bassinet. Is this safe?

It is not unusual for babies to wriggle & move around their sleep environment before they are rolling.
Are you wrapping your baby?
Some babies do not respond well to wrapping & may wriggle to attempt to get out of the wrap/swaddle.
You could try wrapping her with her arms out.
It is important that baby’s head & face remain uncovered & as you are doing, always place baby on the back for sleep.

Red Nose does not recommend the use of sleep positioners or anything that restrains a baby in a specific position.
https://rednose.com.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position