What's the carrier's brand? If you can check for it online, find out if it's safety tested. A lot of non-ergonomic and substandard baby carriers proliferate the market today and they usually cause harm to babies due to accidents.
If it's a mei-tai, the one with two long straps at the base of the carrier, it can be used even by newborns. But if it's an SSC (soft-structured carrier), the one that looks like a back-pack with buckles, it's not yet advisable. This is due to your baby still in her wobbly neck control phase. 4 months should be safe.