diff --git a/crates/ml/src/cuda_pipeline/backtest_env_kernel.cu b/crates/ml/src/cuda_pipeline/backtest_env_kernel.cu
index 07d529c8e..208e0b71f 100644
--- a/crates/ml/src/cuda_pipeline/backtest_env_kernel.cu
+++ b/crates/ml/src/cuda_pipeline/backtest_env_kernel.cu
@@ -137,11 +137,12 @@ extern "C" __global__ void backtest_env_step(
         return;
     }
 
-    // ── Decode action (shared: trade_physics.cuh) ────────────────────────
+    // ── Decode action (4-branch: trade_physics.cuh) ───────────────────────
     int action_val = actions[w];
-    int exposure_idx = decode_exposure_index(action_val, b0_size, b1_size, b2_size);
-    float target_exposure = compute_target_position(exposure_idx, b0_size, max_position);
-    int order_type_idx = decode_order_type(action_val, b1_size, b2_size);
+    int dir_idx = decode_direction_4b(action_val, b1_size, b2_size, b3_size);
+    int mag_idx = decode_magnitude_4b(action_val, b1_size, b2_size, b3_size);
+    float target_exposure = compute_target_position_4branch(dir_idx, mag_idx, max_position);
+    int order_type_idx = decode_order_4b(action_val, b2_size, b3_size);
 
     // ── Margin-aware position cap (shared: trade_physics.cuh) ────────────
     // margin = price * multiplier * margin_pct (e.g. 5000 * 50 * 0.06 = $15K for ES)