diff --git a/tdd_for_embedded_c/chapter04/src/LedDriver.c b/tdd_for_embedded_c/chapter04/src/LedDriver.c
index f3436b9..eb8622e 100644
--- a/tdd_for_embedded_c/chapter04/src/LedDriver.c
+++ b/tdd_for_embedded_c/chapter04/src/LedDriver.c
@@ -1,8 +1,12 @@
 #include "LedDriver.h"
+#include "../util/RuntimeError.h"
 
 
 
-static uint16_t *ledsAddress;
+enum {ALL_LEDS_ON = ~0x0000, ALL_LEDS_OFF = ~ALL_LEDS_ON};
+
+static uint16_t *ledsAddress = NULL;  // LEDs memory register location
+static uint16_t ledsImage;            // internal variable to store LEDs state
 
 
 static uint16_t convertLedNumberToBit(int ledNumber) {
@@ -10,9 +14,18 @@ static uint16_t convertLedNumberToBit(int ledNumber) {
 }
 
 
+static void updateHardware(void) {
+  if(ledsAddress == NULL) {
+    return;
+  }
+  *ledsAddress = ledsImage;
+}
+
+
 void LedDriver_Create(uint16_t *address) {
   ledsAddress = address;
-  *ledsAddress = 0x0000;
+  ledsImage = ALL_LEDS_OFF;
+  updateHardware();
 }
 
 
@@ -21,10 +34,28 @@ void LedDriver_Destroy(uint16_t *address) {
 
 
 void LedDriver_TurnOn(int ledNumber) {
-  *ledsAddress |= convertLedNumberToBit(ledNumber);
+  if(ledNumber < 0 || ledNumber > 16) {
+    RUNTIME_ERROR("LED Driver: out-of-bounds LED", -1);
+    return;
+  }
+
+  ledsImage |= convertLedNumberToBit(ledNumber);
+  updateHardware();
+}
+
+
+void LedDriver_TurnAllOn(void) {
+  ledsImage = ALL_LEDS_ON;
+  updateHardware();
 }
 
 
 void LedDriver_TurnOff(int ledNumber) {
-  *ledsAddress = 0x0000;
+  if(ledNumber < 0 || ledNumber > 16) {
+    RUNTIME_ERROR("LED Driver: out-of-bounds LED", -1);
+    return;
+  }
+
+  ledsImage &= ~(convertLedNumberToBit(ledNumber));
+  updateHardware();
 }
diff --git a/tdd_for_embedded_c/chapter04/src/LedDriver.h b/tdd_for_embedded_c/chapter04/src/LedDriver.h
index 662d639..f09c88f 100644
--- a/tdd_for_embedded_c/chapter04/src/LedDriver.h
+++ b/tdd_for_embedded_c/chapter04/src/LedDriver.h
@@ -1,8 +1,10 @@
 #include <stdint.h>
+#include <string.h>
 
 
 
 void LedDriver_Create(uint16_t *address);
 void LedDriver_Destroy(uint16_t *address);
 void LedDriver_TurnOn(int ledNumber);
-void LedDriver_TurnOff(int ledNumber);
\ No newline at end of file
+void LedDriver_TurnOff(int ledNumber);
+void LedDriver_TurnAllOn(void);
\ No newline at end of file
diff --git a/tdd_for_embedded_c/chapter04/test/LedDriverTest.c b/tdd_for_embedded_c/chapter04/test/LedDriverTest.c
index 987d67e..7242bb0 100644
--- a/tdd_for_embedded_c/chapter04/test/LedDriverTest.c
+++ b/tdd_for_embedded_c/chapter04/test/LedDriverTest.c
@@ -1,5 +1,7 @@
 #include "unity_fixture.h"
 #include "../src/LedDriver.h"
+#include "../mocks/RuntimeErrorStub.h"
+
 
 
 TEST_GROUP(LedDriver);
@@ -54,3 +56,79 @@ TEST(LedDriver, TurnOnMultipleLeds) {
   TEST_ASSERT_EQUAL_HEX16(0x0180, virtualLeds);
 
 }
+
+
+TEST(LedDriver, TurnAllOn) {
+
+  LedDriver_TurnAllOn();
+
+  TEST_ASSERT_EQUAL_HEX16(0xFFFF, virtualLeds);
+
+}
+
+
+TEST(LedDriver, TurnOffAnyLed) {
+
+  LedDriver_TurnAllOn();
+  LedDriver_TurnOff(8);
+
+  TEST_ASSERT_EQUAL_HEX16(0xFF7F, virtualLeds);
+
+}
+
+
+TEST(LedDriver, LedMemoryIsNotReadable) {
+
+  virtualLeds = 0xFFFF;
+  LedDriver_TurnOn(8);
+
+  TEST_ASSERT_EQUAL_HEX16(0x0080, virtualLeds);
+
+}
+
+
+TEST(LedDriver, UpperAndLowerBounds) {
+
+  LedDriver_TurnOn(1);
+  LedDriver_TurnOn(16);
+
+  TEST_ASSERT_EQUAL_HEX16(0x8001, virtualLeds);
+
+}
+
+
+TEST(LedDriver, OutOfBoundsTurnOnDoesNotHarm) {
+
+  LedDriver_TurnOn(-1);
+  LedDriver_TurnOn(0);
+  LedDriver_TurnOn(17);
+  LedDriver_TurnOn(3141);
+
+  TEST_ASSERT_EQUAL_HEX16(0x0000, virtualLeds);
+
+}
+
+TEST(LedDriver, OutOfBoundsTurnOffDoesNotHarm) {
+
+  LedDriver_TurnAllOn();
+  LedDriver_TurnOff(-1);
+  LedDriver_TurnOff(0);
+  LedDriver_TurnOff(17);
+  LedDriver_TurnOff(3141);
+
+  TEST_ASSERT_EQUAL_HEX16(0xFFFF, virtualLeds);
+
+}
+
+
+TEST(LedDriver, OutOfBoundsProducesRuntimeError) {
+  LedDriver_TurnOn(-1);
+
+  TEST_ASSERT_EQUAL_STRING("LED Driver: out-of-bounds LED", RuntimeErrorStub_GetLastError());
+  TEST_ASSERT_EQUAL(-1, RuntimeErrorStub_GetLastParameter());
+}
+
+
+IGNORE_TEST(LedDriver, OutOfBoundsToDo) {
+  /* TODO: what should we do during runtime? */
+}
diff --git a/tdd_for_embedded_c/chapter04/test/LedDriverTestRunner.c b/tdd_for_embedded_c/chapter04/test/LedDriverTestRunner.c
index 89c9674..b558718 100644
--- a/tdd_for_embedded_c/chapter04/test/LedDriverTestRunner.c
+++ b/tdd_for_embedded_c/chapter04/test/LedDriverTestRunner.c
@@ -7,5 +7,13 @@ TEST_GROUP_RUNNER(LedDriver) {
   RUN_TEST_CASE(LedDriver, TurnOnLedOne);
   RUN_TEST_CASE(LedDriver, TurnOffLedOne);
   RUN_TEST_CASE(LedDriver, TurnOnMultipleLeds);
+  RUN_TEST_CASE(LedDriver, TurnAllOn);
+  RUN_TEST_CASE(LedDriver, TurnOffAnyLed);
+  RUN_TEST_CASE(LedDriver, LedMemoryIsNotReadable);
+  RUN_TEST_CASE(LedDriver, UpperAndLowerBounds);
+  RUN_TEST_CASE(LedDriver, OutOfBoundsTurnOnDoesNotHarm);
+  RUN_TEST_CASE(LedDriver, OutOfBoundsTurnOffDoesNotHarm);
+  RUN_TEST_CASE(LedDriver, OutOfBoundsProducesRuntimeError);
+  RUN_TEST_CASE(LedDriver, OutOfBoundsToDo);
 
 }