Witr is
not part of the obligatory Isha prayer.
It is a
separate, independent prayer that is performed
after Isha, and its ruling is that it is
a highly emphasized Sunnah (Sunnah muakkadah).
The scholars of Ahl al-Sunnah are agreed that Witr is
not included within the obligatory four rakahs of Isha. Instead, it is prayed
after Isha, whether immediately or later in the night.
«اجْعَلُوا آخِرَ صَلَاتِكُمْ بِاللَّيْلِ وِتْرًا»
“Make the last of your prayers at night Witr.”
Reference: Sahih al Bukhari: 998
This proves Witr comes after the obligatory prayers, not within Isha.