Week 4-BN: document the working void-return fix (fn.return_value setter)

Y and fn.return_type both fail for _reset_handler; fn.return_value =
ReturnValue(Type.void()) holds through reanalysis (verified live).

WEEK04/WEEK04-BN.md

@@ -619,7 +619,18 @@ main
 | `0x10000da0` | `time_us_64` | `uint64_t time_us_64(void)` |
 | `0x100033e0` | `strlen` | `size_t strlen(const char*)` |
 
-> **A `void` return type may not stick.** Binary Ninja treats `void` as low-confidence, and its analysis can override it with an inferred type — most often `int32_t` on this 32-bit target. It is most visible on `_reset_handler` (a hand-written assembly entry that never returns normally), but it can happen to **any** function whose return type Binary Ninja thinks it can infer. If a header shows a different return type right after you set `void`, that is the analysis winning, not a mistake in what you typed — leave it. It does not affect the rest of the lesson.
+> **A `void` return type may not stick — here is the fix.** Binary Ninja treats `void` as low-confidence, and its analysis can override it with an inferred type — most often `int32_t` on this 32-bit target. It is most visible on `_reset_handler` (a hand-written assembly entry that never returns normally), but it can happen to **any** function whose return type Binary Ninja thinks it can infer.
+>
+> Setting the full signature with `Y` reproduces the unwanted `int32_t`, and `fn.return_type = ...` fails too. What works is the **return-value** setter:
+>
+> ```python
+> from binaryninja import ReturnValue, Type
+> fn = bv.get_function_at(0x1000015c)
+> if fn is not None:
+>     fn.return_value = ReturnValue(Type.void())
+> ```
+>
+> That holds `_reset_handler` at `void` even after reanalysis (verified live). Setting the signature and the return type both failing while `return_value` succeeds looks like a bug or inconsistency in this build (BN 6.0.10601). If it still will not stick, leave it — it does not affect the rest of the lesson.
 
 > **`__wrap_printf` is the real symbol.** `printf` in our source compiles to the SDK's `__wrap_printf` (which forwards to `__wrap_vprintf`). Rename it `printf` if you prefer the lesson's shorthand, but `__wrap_printf` is what the ELF says.
 >