If the mum is your friend it should not be too difficult just to tell her what you think! She might not know what is going on during the playdates and would probably enjoy some time with her 4 year old only when the other child is with you.
Being honest is always the best thing....at least that how it works for me.
I would find it a bit weird if the other child comes as well. And from the mother's perspective, I wouldn't send my younger child to hang out with the older ones. A reason for that could be that she needs a break :).
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