Baby cot bumper

The mesh/breathable type is generally considered safer than the thick padded rectangular ones, since there's still some airflow and you can see baby through it. The "fenced" or vertical slat style bumpers are another option worth looking at, as they slot between the railings and target exactly the gaps where little feet get stuck. Each type has its trade-offs, so it really comes down to your cot design and how active your baby is. That said, since your baby is only 4 months, it might be worth a quick check with your PD too, just to get their take on what's safe for this age.