Firmware 1.8.3 (for hardware 1.x) supports up to 3 expansion boards, and 2.0 (for hardware 2.x) supports up to 5.
Your 1.42u runs on ATmega328 which has very limited EEPROM (1KB). If you want to try 4 expansion boards, you should change the MAX_EXT_BOARDS macro define in defines.h. You cannot run 2.0 firmware on 1.42u because 2.0 is for generation 2 hardware.
You may be able to run up to 9 expansion boards on 2.0, although I have not tried it myself. Does this mean you have close to 80 stations? That’s very impressive 😯
I assume you have OpenSprinkler 1.x hardware. If you increase the MAX_EXT_BOARDS, the program will require more RAM to store the station status and information etc. Unfortunately ATmega328 only has 1KB RAM space, and I am pretty sure increasing MAX_EXT_BOARDS to 4 will cause the RAM consumption to exceed 1KB, causing the mcu to fail to start.