I think Larry and ERTraveller give good advice. Larry nails the ACS perspective. The ACS one is the the one that can impact the flow of federal money. Plus it is easy to compare an apple to an apple
in the ACS designation system.
Now, can the states add additional designations? Maybe, but it could get confusing.
Some states (err..Most) have some type of trauma plan in place. I have no idea if they have their own levels or if they have a state system that details the levels - but... The designation's main function impacts where you (EMS and HEMS) can take certain types of patients.
The OP seems to be interested in the L3 L4 "designation" of the hospital that is the prospective employer. First, I think you will be okay - that is not to say you will not see "some" trauma at the L3 - but, most of "those patients" will never darken your door.
The only time I would stop at a Level 3 out of the helicopter would be with a patient that I could not get to a Level 1 or 2 because of "new" weather that prevented me from reaching my original destination. Also, I might "divert to Level 3" with a massive blunt trauma that goes into arrest with a long ETA to the Level 1 or 2. Read: Rarely, if ever would this happen. Don't worry - the HEMS crew stays and plays if necessary. You will not be abandoned.
Anyway, trauma is a surgical disease. (Actually, I think trauma is a bit easy for the seasoned nurse) Lots of procedures and then off BAM! to surgery in warp time (I hope!). I think this would be a good environment to learn in - providing that you have some resources - experienced nurses, good ratios, an educator, a detailed orientation.