[...]perhaps most likely reason behind Apple's silence, is that the A4 just isn't anything to write home about—and on this second point, I actually know a thing or two. If Apple were to tell you what's in the A4, most of the focus would be on what the chip is not, rather than on what the iPad is.
But it turns out that the the A4 is a 1GHz custom SoC with a single Cortex A8 core and a PowerVR SGX GPU. The fact that A4 uses a single A8 core hasn't been made public, but I've heard from multiple sources who are certain for different reasons that this is indeed the case. (I wish I could be more specific, but I can't.)
Source: Ars Technica