your adapter might be a smps type. in that case if output capacitor is blown then you will not get steady output and enough current. as you said output voltage is 4.3-4.8 varying, it also shows that capacitor is blown. a smps have few key components- switch, inductor , blocking/ fly back diode, output capacitor. inductor failure may cause zero output, switch failure also have the same effect, PWM failure may cause 0 again, diode failure may cause lower voltage (can be same as yours), and capacitor failure have exact same effect due to no storage of power.
if you wanna repair that then you have to debug the whole circuit, bcoz in these kind of devices, failure of one component can fail the another component.
My advice will be to buy a new one. but if you wanna repair that then follow some steps for safety.
i have worked with 3KW smps modules and also faced so many failures as an service engineer. I always replace the faulty component and then cross check every other components related to particular ckt to avoid any future failure. after repair first test the voltage , put the ckt under test load(can be anything not expensive- lihghts, resistors load etc.). only then go with expensive devices.