Place them halfway down in the deck sticking out...and while you're doing this, switch them for a different four of a kind
"I'm going to switch the aces for jacks....see, there goes the first one. Ah, yes, of course I forgot to tell you that to make it even more difficult, the jacks are now tens...oh, there goes another ace!" Mwahaha.
In all seriousness though, asides from the obvious answer about audience management, your fundamental relationship with your audience is what you need to examine here. Most adult audience members faced with an adult professional magician feel a sense of boundary...they may WANT to turn the cards over but they don't because there's a social boundary preventing them - and sometimes even in this situation you get "grabby people". A kid performing for other kids, in a non-professional setting, often has no such social boundary protecting them from grabbers.
If I enounter this kind of person I deal with it in a firm but relaxed manner, making a bit of a joke "hey lady, I'm working here!" or "hey, don't mess with my flow, i'm on a roll" as I place my hand on theirs, preventing the grab. Anyone worth the effort will respect your wishes and leave it - if they get pushy and grab anyway then you shouldn't be wasting your time with them. Of course, the downside to this tactic is that even if it works, you are tacitly admitting that there's something dodgy going on that they can't see, but who believes you're actually magic anyway!
Finally, I would suggest that you may want to re-examine your switch at the beginning of the trick. If the switch is done well, people will NOT suspect the cards on the table, because they KNOW what's down there. What they are much more likely to be concerned with is the cards in your hands - reset is one of those tricks where I get accused of "trick cards" a lot because they assume the cards in my hands are actually changing somehow.
I wasn't joking at the beginning of this post about switching the tabled cards for a different four of a kind either - this makes a great trick and gives you a little more flexibility over the handling.
Cheers,
David.