Yes Soup I've had several, which is why I find some of this far fetched. It's not about the ability to remember a word, it's more a timescale and context thing. Neither am I questioning that some 4 year olds can read prices on a menu and understand that a higher number means it costs more pennies.
I am saying it's highly, highly unusual for a child of that age to even give the concept of cost any consideration when being handed a menu, even if they are ahead of their peers in numeracy.
If her father noticed this and the fact she was no longer asking for treats then why didn't he just ask his DD about it himself? he spends enough time with her. 
I am not saying the OP is a troll or a liar. The worst I would accuse her of is perhaps asking her DD very leading questions and putting words into her mouth in a bid to get the full picture and then getting a bit carried away with what she thinks her DD said happened.
I completely believe that child overheard something about money being given to her mother though, or has had something said directly to her about it.
If she'd come to her mum at the time or shortly afterwards and said 'Mummy what does slut mean?' then I'd believe it. But it didn't happen like that.
Do I believe she heard the word once, weeks ago and has been able to recall it accurately without being led in any way, weeks later? No I don't believe that, sorry.
But I mean it when I say that she has done all these things then she must be exceptionally bright.
The other possibility is that she is used to hearing the word slut and has some understanding of what it means and how it might be used. That would be a different matter altogether.