Interesting. I wonder if as with so many A/C there are several versions. However, I don't understand what you mean by hold 1, 2, 3, 4. The 737 only has 2 holds, i.e 1&4. One thing that I missed was that although both holds are pressurised, neither of them have air-conditioning (737-500). Instead you are given a grid from which you select the initial temperature, and then depending on the flight time you can work out what the hold temperature will drop to during flight. Because of this LH allow live animals in both holds, depending on what else is loaded.