First of all who told you that you should read tasbih after the 4th raka' of Taraweeh?
I was astonished that I've never heard about this until reading your post!
Any act in Islam needs a clear backup, but in this case scholars have three different opinions on thisthe matter some said it is permissibleit is permissible other notnot and others even said it is bida'hit is bida'h among those who said it is permissible you may find scholars like al-Kassani in his book Badai' as-Sanai' بدائع الصنائع from the hanafi school of fiqh and al-Mirdawi -Hanbali scholar- in his al-Insaf الإنصاف . Here the statement of al-Kassani (My own translation take it carefully):
Ibn al-Humam (Hanafi) in his Fath al-Qadir فتح القدير only confirms the sitting or pausing between two sets of taraweeh as a custom of the people of the two harams (Mekka and Medina). So what people have done in between was up to them and that they make tasbeeh etc. is a possible option. He also made clear that people of Medina used to pray during this 4 optional raka'a's.