DeepSeek-V4-Flash's four residual streams are mostly unused.

Measuring effective stream counts, cross-stream residual weights and inter-stream cosine similarity across the four-stream mHC residual pathway shows a typical attention or FFN site effectively uses about two streams, and layers 22-42 mostly carry each stream forward separately (arXiv 2609.05309). Replacing late mixers with identity raises C4 perplexity only 1.9% and preserves the six-task average; replacing early mixers raises it 41%. Fixing each early mixer to its C4 diagnostic mean costs 0.2% perplexity, so site-specific structure matters more than token-wise variation.