IV/4 isn't really IV/4. That is to say, it's not the next bit of CD after IV/3. Rather, it is the bit which Barth had written before he realised that he was never going to finish. He consequently prepared this bit for publication separately, and it stands alone pretty well. It contains Barth's doctrine of baptism, and it is (to my mind) intriguing and controversial.
The book is split into two uneven parts. The first, shorter, part deals with baptism in the Spirit. This is the answer to the question 'how does the objective truth of reconciliation to God in Christ become subjectively real for me?' - a hugely important question at all times, and especially I would suggest in our time. Barth's answer is typically Reformed, with all the stress being on God's initiative. One emphasis which perhaps does not come across so much in other Reformed writers is the freedom of the human being who is baptised with the Spirit - not freedom to accept or reject this baptism, but freedom as a result of this baptism. By the power of the Spirit, a person is set free to walk in the way of faith in Christ. That means they are not free to do anything else, because for Barth freedom is not libertarian freedom; it is freedom to serve and trust God.
In the longer second part Barth deals with water baptism. He builds on the understanding of freedom in the first part to argue that baptism in water is the first free act of the newly freed man. It is requested from the Christian community, which administers it because it is called to do so by Christ. In so doing, the community recognises the baptised as a fellow Christian, and the baptised acknowledges the community. They commit to standing in solidarity of witness.
What is perhaps most controversial about Barth's account is his insistence that nothing sacramental occurs in water baptism. In the face of almost all church teaching through the ages, Barth argues against the idea that there are two subjects in baptism - the community which baptises, and God who acts sacramentally through the community's action. Baptism in water is, for Barth, a wholly human action, a response to the freeing presence and action of God the Holy Spirit. Partly this is driven by Barth's theological concerns - for the freedom of God, who is not bound to the community's actions, and for the freedom of the Christian, who is set on the path of obedience by the Spirit - but it is also backed by a lot of exegetical work. Barth asks the question: 'where in Scripture is baptism described in a way which implies a sacramental understanding?' - and finds no evidence to support this understanding. (To be more precise, he finds passages that could be construed in a sacramental way, but others which point decisively against it). I was also intrigued - and this is a challenge to my own previous thought - that Barth traces Christian baptism primarily back to Christ's baptism in the Jordan, making this the interpretive key for the whole doctrine.
Barth's doctrine, which deserves to be much more fully expounded than the hints I have given here, leads him to reject infant baptism, with a series of entertaining and incisive arguments which I recommend any of my paedobaptist brethren to take a look at - although doubtless they will not be persuasive if you don't buy into his whole picture. Predictably, I rather enjoyed this section.
So, here's my question - where does the idea of a sacrament come from, Biblically speaking? I am on the verge of saying, with Barth, that the only sacrament - understood as a divine action accompanying/underlying a human action - is Jesus Christ himself. Would I be catastrophically wrong to say so?