Hi there with regards to the iphone SE battery the voltage on it should NOT be 4V it should be only around 3.8V and yes 0.2V do make quite a significant difference with Iphones! There is a line on the iphone which is very picky and if that line is even 0.1V lower it will not work. First I would like to ask if the phone does or doesn’t turn on? Next I would like to ask you to measure your battery again without it plugged into the phone and tell me your measurement here. If the phone does not turn on and the battery still is stuck at 4V I would personally suspect it to be the problem and would try and replace it as part of diagnosing the problem. If you don’t have a spare battery to hand you can perhaps try and see if the main power lines on the phone are present, try this by unplugging the battery and plugging in the phone into a charger. Measure the main power lines and see what you get if they are present most likely its either a hardware issue or a board problem however a board problem relating to the...
