Will this model replace my existing BRK 4120B model?

All smoke detectors will work if they are made for 2 and 3 wire systems. First compare the plug on the new one with the existing. If they are not the same you will have to install the pigtail with plug that came with the new one. Follow instructions, match wire colors and be sure to turn off the electricity to the fire alarm system before you get to work. They are all usually on one circuit, but not always. Check for power before proceeding. You will have to also change the mounting ring, also included with the new detector, as they rarely match. Once you have done these two things turn the power back on and check for correct operation. Also, remember to put the backup battery in as well.