Will iPhone 5S be Free When iPhone 6 Comes Out?

Currently, Verizon offers the iPhone 4S for free (with two year contract) while the iPhone 5S is $199+.

When the iPhone 6 comes out this fall, will the iPhone 5S immediately become free while the 4S gets phased out?

It is a possibility, but not guaranteed. The 5S is still a relatively new phone, chances are it will be reduced in price first, as the 4S was when the 5 came out. The iPhone 4S has been out since 2011 and has just recently been phasing out.
Phone companies may offer promotions around Christmas time to get the 4S for 0$.
I purchased my iPhone 4S in December of 2012 for $0 on promotion, went back to $99 after Christmas. While my boyfriend got the 5 (weeks after release), also in December 2012, for $79 on promotion, which went back up after Christmas.

Let me check my magic crystal ball.