Weddingbee boards

Well it depends on the tone and formality of the rest of your invitation wording, but this might get you started:

"Please join us again at [venue] at six o'clock for dinner and dancing."

 I'm no etiquette expert, but that seems pretty clear.  There's just no polite way to say "scram for 4 hours!" :-P so putting that word 'again' in there should imply that they'll be leaving in the meantime.

 Here's an extra thought--you might consider listing a few ways/places that your guests can entertain themselves in the interim...a lot of them probably won't want to go home, and they'll all be dressed up (I assume) , so you could possibly list some museums, gardens, etc. that they could use to fill their time.

And the more I think about it, I think that your program (if you're having programs) would be a good place to list this info...then you could be more playful, and wordier.  Something like "Until we see you again at six o'clock for dinner, here are a few things to do in our fair city..."  Only, of course, with your guys' own personalized wording and everything.

HTH! :-)