The waters around Martinique were extremely shallow and Hartenstein decided not to pursue the destroyer. On 1 June 1942, Hartenstein was promoted to Korvettenkapitän (corvette captain). The first watch officer, Paul Just, left U-156 after returning from the third patrol. Just was replaced by Oberleutnant zur See Leopold Schumacher as new first watch officer. Just later became commander of , and . Hartenstein and the entire crew of U-156 received a hero's welcome by the residents of Plauen on 20 July 1942. The people lined the streets as the whole crew marched from the railway station to the City Hall for the official welcome reception. Fourth patrol and Laconia incident On U-156's fourth patrol (20 August 1942 – 16 November 1942), Hartenstein sank and then organised the rescue of the survivors of RMS Laconia, resulting in the "Laconia incident" and "Laconia Order". U-156 together with under the command of Karl-Friedrich Merten departed from Lorient on 20 August 1942 heading for the Bay of Biscay. Hartenstein received the order to operate against Convoy SL-119 on 25 August. After a two-day pursuit, U-156 found a straggler, the SS Clan Macwhirter, west of Casablanca. Hartenstein attacked from a submerged position to avoid detection under the bright moonlight. Clan Macwhirter was hit by two torpedoes and sank, killing nine members of the crew and two gunners; 79 sailors survived the sinking. Two later sightings on 2 and 6 September did not lead to favourable attack positions. On 12 September 1942 U-156 was patrolling off the coast of West Africa midway between Liberia and Ascension Island roughly south of Cape Palmas. At 11:37 the aft port lookout sighted a smoke stack at 230 degrees. Hartenstein followed the target, which was zigzagging at , until the general direction of the large ocean liner became evident. U-156 was running at into a favourable attack position, and Hartenstein ordered the attack at 21:07. He slowed speed at 22:00 and ordered surfaced deflection shots from torpedo tubes I and III. After three minutes and six seconds the first torpedo detonated, then the second. He had hoped to capture the ship's senior officers, but to his surprise, Hartenstein saw over two thousand people struggling in the water. Hartenstein immediately began rescue operations. Laconia sank at 23:23. At 01:25 on 13 September 1942 Hartenstein radioed the Befehlshaber der U-Boote (BdU—commander of U-boats) requesting guidance and confirmation on how to proceed. The