The answer isnt simply that 4 trucks is too much. The total number of packages, their size and weight, where you are on the belt, the length of the shift and probably even more factors all matter. For the average package size somewhere around 200 an hour is about the maximum that anyone should be doing, and 175 an hour is ideal. If you load an unusually high number of smalls then you can do a little more. To determine what a preloader is able to safely load, the total effort required to get the packages to where they are suppose to go on the truck should be the focus. Ideally what should go into this calculation is the size and weight, so instead of counting number of total pieces, the calculation should be the total size and weight of the packages. Obviously there should be a hard limit if all a preloader loads is smalls, but you get the idea. Customers have to enter this information into the form when printing a shipping label, but for some reason this information isnt closely monitored. Other factors that should be considered are things like does the preloader have to step up into a 24 footer or one of the other trucks that you have to step up into, is it a hot stuffy time of the year or are temperatures and humidity ideal for physical activity. I know most of these factors will never be considered when determining what a preloader is capable of doing, but they do matter and they should be considered.
For the OP the best answer I can give is try doing the 4 trucks. Work at a reasonable pace but stay under control and work safe. If you find that you are not able to keep up with the work then bring it up with your steward. If your steward agrees that the work is too much to safely do, then he/she should be having a talk with management to get your work load reduced. I have the benefit of a good steward in my building that will stand up for their fellow teamsters. Maybe you don't have a good steward that seems to care less about your safety. If thats the case then I would contact the local teamsters office and tell them that your steward is not supporting their teamsters.