Its less being well known on the forums, and more contributing to the community.
1-3: We aren't aware about how contributions are converted into IP. This hasn't been detailed.
4: Influence points aren't spent. Instead they determine priority in matters of selection, creations, and eventually design. As an example, someone with 5000 IP would get priority in domain selection to someone with 4500 IP. Though the mechanics for this have yet to be detailed.