The broadcast pattern for TAR 4 will follow that of TAR 1, and not TAR 2 and 3.
Leg 11 will air August 7, Leg 12 will air August 14, and the final leg (leg 13) will air August 21.
Someone contacted World Race Productions and confirmed the August 21 date a couple of weeks ago. And the August 14 episode title was posted this morning at a couple of places, so this scenario seems to be accurate.
As to how much of a lead Reichen and Chip have, that's been a matter of intense speculation elsewhere. It may be as much as 4 or 5 hours, depending on how early Reichen and Chip finished the last leg (possibly before noon), and how long it took Kelly and Jon to reach the pitstop (mid to late afternoon).