Lemon may need some extra time to adjust to the latest changes and to find her place on the recently created flock. While you got your budgies from the place, Lemon may not have been acquainted nor close to Camellia, Blueberry and Lime while at the store.
You mentioned on your introductory thread that you got your flock a bit over a week ago. At this point they are all still adjusting to their new home and family.
The fact that they are all exploring the cage and starting to play with their toys is great.
As for Lemon, unless she is showing clear signs of being stressed and uncomfortable by her flockmates and this in itself is negatively impacting her mental welfare, then you won't need to take a more radical approach to the situation (by housing Lemon on a separate cage).
Keep on observing the behaviour and interactions between your budgies while having in mind the real possibility of having to separate them later on.
Depending on how things progress with Lemon, on whether there is mutual acceptance from her and the rest of the flock, there are a few different scenarios that can come into play. I will proceed with a couple examples that would demand separate housing arrangements.
For example, Lemon can be completely ignored by the rest of your flock and in the process your two boys can focus more of their attention on Camellia and as they grow and mature, this could lead to trouble and fights for the female's affection. Another less than desirable scenario would also be for one or more of your budgies to gang up on the odd one out, Lemon.
As time goes by, you will have a much better understanding on your budgies' personalities and on how they function as a flock.
Depending on the developed bonds, you could also pre-emptively house each pair in a different cage in order to avoid future problems when they first go into breeding condition.
Also mirrors/mirrored toys can do more harm then good, this is especially the case when a given budgie develops an unhealthy attachment to the reflection on the mirror.
The information on this link may be of use: https://talkbudgies.com/articles-gene...en-flocks.html