LDO3/4 won't turn off when the battery is below 3.5V
You simply have to account for the I*R drop across the LDO. For example, if the load current is 100mA and the resistance of the LDO is 0.4 ohms, then the I*R drop would be (100mA * 0.4ohms) = 40mV. In this case, you would need to have 3.34V into the LDO in order to get 3.3V out.