The s2m_x4 kernel is generated from the kernel/spec.json specification. Review this file again. Notice that the s2m_x4 kernel has an implementation StoreStreamToMaster which moves data from AXI-Streams to DDR. The s2m_x4 kernel has 4 channels: k0,k1,k2, and k3. Each stream writes the data coming from the tx0-tx3 streams to a DDR buffer.