Final answer:
The Ural Mountains are the physical feature closest to number 4, acting as the traditional boundary between Europe and Asia and separating the Northern European and West Siberian Plains.
Step-by-step explanation:
The physical feature closest to number 4, given the context of the information provided and the listed options, is the Ural Mountains. The Ural Mountains are known for separating the Northern European and West Siberian Plains, and they serve as the dividing line between the European and Asian continents.
The European Plain is an extensive area of flat land that spans across many countries in Europe, and it's recognized for its agricultural productivity. However, given that the other options like the Alps and Pyrenees are also part of the physical landscape of Europe but are not closest to the marker of number 4 on the given information, the Ural Mountains, which extend from the Arctic Ocean to the Ural River and into western Russia, are the correct feature associated with this number.