The regulator needs a capacitor on it's input. Read the datasheet - they usually have an example circuit. But this may not explain the waveform you are seeing. It could also be your ground reference for the scope. The scope ground should be on your regulator REF pin. Also, you don't identify the IC1 device, so no idea the characteristics of that pin you are sensing with. Sometimes this can be caused by differences in the diode forward drop characteristics, but 4 volts is too much to be explained by that. You will likely be exceeding IC1 input voltage, possibly triggering internal protection circuitry, and so on. Your 10K and 1K probably need to be reversed,