Can I store all the files in the directory alf_data in Amazon S3?
Yes, if you are using a content store implementation / addon that supports S3, you can technically store every item in the contentstore directory in S3. Ideally, you would likely use a caching content store to avoid accessing S3 multiple times for a specific file (to reduce latency and cost for access), so the cached content items may still be stored in contentstore after all.
And solr4 data I can store in s3?
You do not want to store anything else (other than contentstore) in S3. Storing SOLR data in S3 will significantly degrade the performance of your system. That data is accessed extremely frequently and requires very low latencies for proper performance (usually SSD is recommended for larger data sets).
Do I need to back up the solr data or can I always re-index it?
The documentation says you can always reconstruct an index from the database state. It does not lie in regards to that...
Retrieving data ...