I think no.1 is the cleanest (though still needs simplifying and a better font choice) but I like that no.4 doesn't use a lightbulb.
No.2 is has a very tall footprint which makes it seem imbalanced. No.3 will lose legibility at smaller scales (that light bulb won't be obvious).
1. Reveal speaker lineups or special guests in a creative way
2. Post often about early bird deadlines and registration windows
3. Perform giveaways for those with FOMO
4. Share behind-the-scenes pics waaaay before the event starts
5. Have a photo booth