I don't quite understand how exactly Multistream single-index rendering works. I've had a look in the example that comes with the sdk but I still can't get it to work.

I have 4 vertex buffers (ID3D10Buffer* vb[4]);  In vb[0] I store vertex position (for the sake of simplicity I store everything as D3DXVECTOR4), in vb[1] texcoords, now I'm not really sure what to put in vb[2] & vb[3]. The way I saw it in the example is that they use a D3DXVECTOR3 value, so what should I store in those 3 values??

The number of streams depends on your vertex format and your needs. If you have only positions and texture coordinates you will only need 2 streams.

Beside of learning how it works you should always try to use as less streams as possible for performance reasons. In general vertices are only spitted to multiple streams when they are used for different render passes that consume different data’s. Like a shadow map pass that only needs the position and the scene pass that needs all.