Can I suggest you go for a wheelchair sooner, rather than later? DD is 3.3 and a wobbly walker - mild Spastic Diplegia. We were referred to wheelchair services in January for a SN buggy as the pushchair was beginning to look a bit snug (interesting, as she is tiny really). We looked at the Mac Major, but didn't like it at all, and the other buggy we were shown was the Swifty. It was ok, but I wasn't a great fan. Then I asked about a wheelchair and the WCS person nearly fell over in surprise - she said parents usually don't want to consider a wheelchair so young.
But, do you know what? DD loves it. She very quickly decided she didn't want to use her pushchair any more, and we now use the wheelchair whenever we go out. The reason we went for the wheelchair is because I think other parents and children are far more accepting and find a wheelchair far easier to understand than an older child in a pushchair. DD's language skills are very good, so she'd understand all the 'Why's that big girl in a baby's pushchair?' type questions. A wheelchair is far easier to understand and for parents to explain - she is in the wheelchair because her legs don't work properly. She is also starting to self-propel. so has a small measure of independence.
Also, out and about, where most people would ignore DD in the pushchair, when they see her in the wheelchair they smile, chat and are so helpful - opening doors, carrying things, etc. Also, when they see DD wobbly walking and me pushing the wheelchair, they smile and say 'Isn't she doing well!' rather than 'Oh, a new walker?' (Not being funny, but we also get the disabled rate for days out which often come with a free carer ticket - not that this was our motivation, but is a nice side effect!)