Starting solids for breastfed baby

Charlotte 🇬🇧 • Mummy to 2 beautiful little girls. Born 2016 and 2018.
My LO has just turned 4 months, she's exclusively breastfed. Some advice says start introducing purees at 4 months others say wait till 6 months. Most agree that you should wait until baby can sit up. But does this mean unaided or in something like a bumbo or high chair? I don't think she's ready just yet but I'd like to be prepared. Any advice/ opinions would be great :)