Doubt in the number of the units (Rak'ats) has different types, some of the doubts are irreparable and Salah is invalid if doubt remains like doubt during morning and Maghrib Prayers and doubts in the first two Rak'ats in any Prayer.

There are repairable doubts  which are during any of the four units Prayers (Dhuhr, Asr and Isha) if it happens after completing the second Rak'ah.

Your doubt between third or forth Rak'ah is repairable. You should consider your Rak'ah as the forth and complete your Prayer then stand up for just one Rak'ah of Salatul Ehtiyat which is just reciting Sura Al-Hamd then Ruku' then two Sajda then Salam.

You wrote that you performed another unit. That made the number of units reach to five which is incorrect.

You should repeat that Prayer to be sure that you fulfilled a proper Prayer of four units only.

'Wassalam.