There is an unsupported way to double down save an InDesign CS4 Document to CS2. Have a look at the following link
Another way would be the long route
1. Downsave CS4 to CS3 in INX
2. Edit CS3 Document for any required changes
3. Downsave CS3 to CS2 in INX
4. Edit CS2 Document for any required changes.
If you are missing CS3 installation, then double down save could be a solution for you. If you need help, we can help you.
- Amit Gupta