Hakim al-Umma Hadhrat Mawlana Ashraf ‘Ali Thanawi (may Allah have mercy on him) said,

“There is no fixed form of dhikr in the interval between each four units of taraweeh(tarweeha) as per Shariah. I recite salutations on the Prophet of Allah (may Allah bless and give him peace).  In my personal opinion this is best. I recite it twenty five times (in a soft audible voice) so that if anyone has to do some necessary activity, like drink water, he will be able to rejoin the prayers (salah) before we restart.”