Nowhere are the Upanishad exlaining the 4 Vedas.
The first chapters of the Brihadaranyaka Upanishad is an explanation of the Ashwamedha Yagya. ^_^
While your argument is sort of correct, it's also sort of incorrect. The Vedic Samhitas are hymns devoted to various gods. Books 2-7 of the Rig Veda came from different families, so of course there's going to be some contradictions within them, let alone beyond them. The samhitas aren't meant to be religious teachings; they're just hymns for the most part. They contain the beliefs of the authors, but that's not the same thing. Therefore, we can't determine exactly what the Vedic religion was based only on the Samhitas. Even the Brahmanas are insufficient due to the fact that they are just manuals for the yagyas.
The Atharva Veda is in a class all on its own; the later Vedic hymns and the parts of the Brihadaranyaka Upanishad (which, BTW, is one of the oldest ones and is attached directly to the Shatapatha Brahmana, the Kanva recension, I believe, of the Sukla Yajur Veda), only speaks of three: Rik, Saman, and Yajus.
Just as modern Hinduism is really an umbrella of many religions, it's clear to me that Vedism also represents many religions, either based on region, varna, and ashrama. Therefore, one region's interpretation may have believed in only one life on earth, while a neighboring region would say that we have many. In either case, the latter is what would become the dominant belief, and is primarily what the Buddha was reacting to.