This is not a 'bug', but it is an issue that could be addressed.
Of course i know better than to try converting any ul games to ISO from a FAT32 formatted drive, especially when the game is more than 4GBs. However, i did it by accident today and found there is no error message or any way to stop the process from continuing. So after it completed its conversion, the Command window simply closed afterwards and left behind a 3.99GB ISO, which to the average noobie, would assume its a good ISO to use now. The real ISO is 4.33GBs and something was left out in the conversion process, leaving you with an ISO that will not work properly.
My suggestion is to add in some method to detect if all ul parts for the selected game are over the 4GB limit and detect if the destination drive is FAT32 or not. If it detects the FAT32 file system, it could give users an error message and halt the process if the parts exceed the 4GB limit.
Or possibly allow to choose your own destination, since many people who might wish to convert have there games stored on a FAT32 drive in ul format
I dont know whats possible, but i figured i toss this out there and let you devs decide if anything can be done to help improve opl2iso.