ER orientation....

Sure it's legal. There's no law in the US that stipulates how many hours a nurse needs to be on orientation before being on their own, whether it be ER or any other specialty. Orientation durations are at the discretion of the employer. However, is it ethical for an employer to place a new grad RN on their own in an ER after only 4-5 shifts of orientation? Probably not.

As for the risking your license part, the fear of new grads losing their licenses is waaaaay overrated. The majority of disciplinary actions taken against nurses are usually for things like diversion, impairment, substance abuse, falsification of records, etc. and rarely for patient care related issues. You'd probably have to be extremely grossly negligent to have your license revoked for a patient care issue. But that in itself is another discussion that I would rather not get into here.

In summary,

yes that type of orientation is unsafe for an ER,

yes facilities can offer such orientations with nothing being illegal about it,

the hospital can justify it as long as nothing has gone wrong yet and people are willing to put their patients' safety at risk by accepting jobs like that,

no you are not likely to lose your license over taking such a position, but

yes you are more likely to cause harm to a patient and be thrown under a bus at that job versus starting at a different facility.

Your best course of action is to accept that this job is unacceptable and to keep searching.

It depends. Some organizations have a (multi-phase) "layered" orientation model. Phase one is just survival skills -policies, routines, etc. Phase 2 is focused on basic skills, Phase 3 is advanced skills ... and so on. So, what are the orientation goals/objectives? If the goal is to simply learn the routines and processes of ED, that wouldn't take very long. If you are expected to assume independent patient care, it's probably not long enough.

OTOH, the only thing that will jeopardize your license are the decisions YOU make. Like assuming responsibility for tasks/skills that are beyond your scope of licensure or competency. Or - continuing to work for an organization that makes you do this. Our BONs expect us to "just say no" if we're faced with a situation that is in violation of our Nurse Practice Act. Sometimes this may mean loss of employment. You can always get another job... but licenses are one to a customer.

This is why it is important to ask an interview, "Can you tell me about the training involved for this position? What sort of training is provided on/off the floor and for how long?" If the unit/area/hospital does not have a formal training process (example: shadow for X, work with preceptor for Y, take on your own patients at Z point in time, etc.) I would keep looking at other jobs.