In the context of the question, the correct answer is option 4, Pyinmana. Pyinmana was announced as the new administrative capital of Myanmar in 2005. The city has undergone substantial development to host the country`s administrative functions. Now, the area including and surrounding Pyinmana is often referred to as Naypyidaw, the new capital of Myanmar.

Option 1, Bassein, which is also known as Pathein, is a port city in the Ayeyarwady Region of Myanmar. However, it hasn`t been proposed as a new administrative capital.

Option 2, Mandalay, is the second-largest city and the last royal capital of Myanmar. Although it holds significant history and importance, it isn`t the new proposed administrative capital.

Option 3, Myithyina, is the capital city of Kachin State in the northernmost part of Myanmar. However, it is not the newly proposed administrative capital.

Therefore, the correct answer is Pyinmana (or Naypyidaw) as per the question`s scope.