What is the ruling on doubting, during the 'Asr prayer, whether one has performed three or four rak'ahs, especially if this doubt has occurred repeatedly in the past, given that the questioner based their action on the greater number? And does doubt affect the validity of the prayer if it was interrupted and restarted? And is it obligatory to pronounce the Two Testimonies in this case?

What you mentioned about doubts in prayer and disbelief is a result of waswasah (whisperings/obsessive thoughts). You should, therefore, avail yourself of the rukhsah (definition) (concession) for those afflicted by waswasah, and completely disregard these whisperings. This is because indulging them leads to confusion and illness. The effective cure for waswasah is to completely and utterly ignore it, believing it to be a satanic thought and fighting against it.