In the name of Allah, Most Compassionate, Most Merciful,
Jazakum Allah khayr for your question!
It is sunna to send blessings upon the Messenger of Allah (Allah bless him and give him peace) after two rakats that are (a) voluntary prayers that (b) are not the emphasized sunna (sunna mu’akkada). [Shurunbulali, Maraqi al-Falah]
Thus, if you are praying the four non-emphasized sunna rakats before Asr prayer, you would recite send blessings on the Prophet (Allah bless him and give him peace) after every two rakats. This is because every two rakats of the voluntary (nafl) prayers are considered as a prayer on their own.
However, in the emphasized sunnas—such as the four rakats before Zuhr prayers—you would only send blessings in the final sitting of the prayer. This is because the emphasis on these prayers makes them resemble the obligatory prayers. [Shurunbulali, Maraqi al-Falah]
And Allah knows best.
I hope that the above answers your question. If something is unclear please don’t hesitate to ask again.
May Allah Most High grant you the best of this world and the next. Please keep us in your du’as!