According to the q33 schedule (which might be out of date), their first bus from 74th street station on the #7, F, E lines stop, departs at 4:05 am, and the 2nd is about an hour later 5:00 am (20 minute ride) these will get you to lga within your constraints. Security at lga (at least the main terminal) doesn't open until 4:30 am. This station is a major hub and i don't think you run into safety problems. Riding any of the lines noted above in the car with the conductor, should add an extra measure of safety.
I've never been at that station at 4:00 am, but I would assume that taxis would be available--the ride is so short, that the fare would be very low.