Use 4096-bit DH prime, equivalent to AES-128

We're using DH over a finite field modulo a prime, not ECDH. To have the same strength as a 150-bit symmetric cipher key, against the best attack techniques 2004 had to offer, we'd need about a 4575-bit DH modulus.[1] AES-128 is about as hard to break as a 3200-bit DH modulus given the best techniques of 2001.[2]

[1] https://tools.ietf.org/html/rfc3766 (see table in section 5)
[2] http://tools.ietf.org/html/rfc3526