docs: add DeepSeek V4 FP4 indexer usage (#27035)

Co-authored-by: Jinyan Chen <jinyanc@nvidia.com>

docs_new/cookbook/autoregressive/DeepSeek/DeepSeek-V4.mdx

@@ -156,6 +156,18 @@ MegaMoE is not supported with this DeepEP Waterfill recipe yet. Waterfill routes
 the shared expert through DeepEP for load balancing, so `--enable-deepep-waterfill`
 requires `--moe-a2a-backend deepep`.
 
+**FP4 Indexer (Experimental)**
+
+DeepSeek-V4 uses the default indexer path unless `--enable-deepseek-v4-fp4-indexer` is set. Enable this flag to use the experimental FP4 C4 indexer on SM100 GPUs with DeepGEMM FP4 indexer support. This path is intended for decode-heavy long-context workloads where reducing indexer cache bandwidth is beneficial.
+
+```bash Command
+# Please use latest main branch for this feature
+sglang serve deepseek-ai/DeepSeek-V4-Flash \
+  --tp 4 \
+  --moe-runner-backend flashinfer_mxfp4 \
+  --enable-deepseek-v4-fp4-indexer
+```
+
 <a id="hopper-note" />
 
 **Hopper (H200) note**

docs_new/docs/advanced_features/server_arguments.mdx

@@ -1218,6 +1218,12 @@ Please consult the documentation below and [server_args.py](https://github.com/s
       <td style={{padding: "9px 12px", backgroundColor: "rgba(255,255,255,0.02)"}}>`sgl-kernel`</td>
       <td style={{padding: "9px 12px", backgroundColor: "rgba(255,255,255,0.05)"}}><code>sgl-kernel</code>, <code>torch</code>, <code>flashinfer</code></td>
     </tr>
+    <tr>
+      <td style={{padding: "9px 12px", fontWeight: 500, backgroundColor: "rgba(255,255,255,0.02)"}}>`--enable-deepseek-v4-fp4-indexer`</td>
+      <td style={{padding: "9px 12px", backgroundColor: "rgba(255,255,255,0.05)"}}>Enable the experimental FP4 C4 indexer path for DeepSeek V4. When unset, SGLang keeps the default DeepSeek V4 indexer path. Requires SM100 GPUs with DeepGEMM FP4 indexer support.</td>
+      <td style={{padding: "9px 12px", backgroundColor: "rgba(255,255,255,0.02)"}}>`False`</td>
+      <td style={{padding: "9px 12px", backgroundColor: "rgba(255,255,255,0.05)"}}>bool flag (set to enable)</td>
+    </tr>
     <tr>
       <td style={{padding: "9px 12px", fontWeight: 500, backgroundColor: "rgba(255,255,255,0.02)"}}>`--fp8-gemm-backend`</td>
       <td style={{padding: "9px 12px", backgroundColor: "rgba(255,255,255,0.05)"}}>Choose the runner backend for Blockwise FP8 GEMM operations. Options: 'auto' (default, auto-selects based on hardware), 'deep_gemm' (JIT-compiled; enabled by default on NVIDIA Hopper (SM90) and Blackwell (SM100) when DeepGEMM is installed), 'flashinfer_trtllm' (FlashInfer TRTLLM backend; SM100/SM103 only), 'flashinfer_cutlass' (FlashInfer CUTLASS backend, SM120 only), 'flashinfer_deepgemm' (Hopper SM90 only, uses swapAB optimization for small M dimensions in decoding), 'cutlass' (optimal for Hopper/Blackwell GPUs and high-throughput), 'triton' (fallback, widely compatible), 'aiter' (ROCm only).</td>