When you are blinded, you first switch on the emergency lights, then, without changing lanes, slow down and, if necessary, stop. The high beam is switched to low beam so as not to blind others, not when you have been blinded.
Under the rules (clause 129), when blinded by the headlights of an oncoming vehicle, the driver must immediately switch on the emergency lights. After that, without changing lanes, the driver must reduce speed and, if necessary, stop to avoid an accident.
Option 1. Switching to low beam is required so as not to blind others, not when you have already been blinded.
Option 3. Giving a sound signal is the wrong action when you are blinded: you must switch on the emergency lights.
Option 4. The rules provide no sound signal at all for being blinded: you need to switch on the emergency lights.