Explanation:

- Highly dissected topography: The hilly and mountainous terrain makes it challenging to lay railway tracks.

- Dense forests: These make construction and maintenance of railway lines difficult.

- Low population density: While it may be relevant for economic viability, it is less a direct factor in the technical or geographical challenges of construction.

- Heavy rainfall and frequent flooding: This complicates infrastructure maintenance and construction, disrupting railway services.

Considering these explanations:

- Option 1 includes population density, which is less impactful.

- Option 2 and Option 3 miss the critical geographical and climate factors.

- Option 4 correctly includes the most significant obstacles like topography, forests, and climate.

Option 4 is the correct choice.