At 4+ I am not sure how much "academics" matters. Its more about a child starting to enjoy education, by feeling safe, engaged and so on and developing good study habits, social skills and resilance.
There are big differences between the two schools and I would think a bit about the next stage. From its website Knightsbridge seems to offer the full range of destination schools, though you might ask for specific numbers. One child only to SPGS in three years might suggest they are not always getting the right child into the right school. I think though it is equally as good that other children go to schools that are right for them. Not all will thrive in very academic environments.
Queens Gate is a good school, but small, all girls and nurturing. Girls who go there often do very well, but it depends on the child.
I would think carefully about your child. If they are more boisterous and would thrive in a co-ed environment, then Knightsbridge. If not Queens Gate. I would also factor in that Knightsbridge will prepare for 11+ and 13+. As Queensgate has a senior school they may not prepare as well for 11+. You should ask them about this. There is sme anecdotal evidence that it is easier to get into a co-ed secondary from a co-ed prep, but since there are a lot more girl's secondaries this may not be an issue.
And do try to get a feel for fellow parents. As Knightsbridge is fairly new I would not expect them to be picking up many of the trad posh. Ditto Ken Prep, Falkner House, Glendower and Bute will be the target schools for the super ambitious. But I don't know enough to answer your origional question. There are plenty of more relaxed parents who take education seriously but will be looking for schools that feel right for their small children, and Knightbridge may be one of them. But equally it may be the go-to school for the scions of London socialites. I assume you could ask the school about other parents and what they do. When we were at this stage we were very reassured by a Head (of a prep just south of the river) who claimed they had a number of parents who were pretty stretched but had prioritised their children's education.