Concept:
Biosphere reserves are protected areas established for conservation and sustainable use of biodiversity.
Step 1:
A biosphere reserve generally:
• Conserves biodiversity
• Protects endangered species
• Supports research activities
Step 2:
Some regions inside biosphere reserves may include human settlements and modified ecosystems.
Step 3:
Thus saying it must not contain any man-modified ecosystem is incorrect.
Step 4:
Hence NOT essential characteristic is:
\[
\boxed{\text{It is not a man-modified and degraded ecosystem}}
\]
\[
\boxed{\text{Correct Option = (4)}}
\]