What are my options to join the Guard after going through a medical retirement and receiving a full honorable discharge with an RE4 code?

Medical separation, yes.
Medical retirement, no.

You may want to look into something like the National Guard Youth Challenge Program, You wear the uniform in many cases and work with Teens from 16 to 18 in a live in Military environment but you do not have to meet the Military Requirements of PT or deployment medical fitness, Essentially you use your military background and experience in a civilian state Position that falls within the National Guard Umbrella

SFC (Join to see) probably has some great advice on this one

Hey man. I feel you and completely understand. I got a Med Retirement, too. I want back in but in out circumstance, the short answer is no. RE Code 4’s are very difficult to get to reenlist. First thing, you’d need an Exception to Policy which is hard to get to begin with. But someone mentioned the Youth ChalleNGe Program. Those are great programs and get to be in that military environment.

This seems counterintuitive to me.
If your body is so broken they kicked you out because you don't meet medical retention standards, then doesn't it logically follow that you would continue to not meet retention standards if you accessed in another branch? Wouldn't they be obligated to kick you out all over again since you don't, and continue to not meet retention standards?

RE-4 + back injury= never going to happen. No MEPS CMO will let you walk in the door with a spinal injury recovered or not. The other gentleman had it right, focus your energy on something else fulfilling like community service.

Try getting a civilian job with the Corps of Engineers. One of my child hood friends has worked for them since she was a teenager and one of my FB friends has been working for them since he retired from the Army. Both of them love working for the Corps of Engineers. Good luck.

I'm a little tardy to the party on this particular feed, and I'm not going to jump on to provide the same feedback that everyone else has given, but I will provide an alternative. Does your state have an active State Defense Force? They have much less stringent medical qualifications and are typically an all volunteer, unpaid force that falls under the State's Military Department. Their mission keeps them within state borders and is meant to augment the Army and Air National Guard during times of emergency. You receive pay and benefits at the level of your rank when activated for state active duty. My understanding is that it operates very similarly to ARNG and ANG, in that you have monthly drills, wear the service uniform with certain differences to distinguish from the federal services, and ply your knowledge and expertise to better your state and your community. It may not be exactly what you were looking for, but it may be about the closest thing you can do to match your prior Army time.

SFC Pruitt is on target.
Check out your state's National Guard ChallenNGe Academy.
It is great job if you like mentoring youths.

Short answer that you probably won't like.
You are not going to be going back to wearing the uniform as a member of the military.

Look at options like working with a local FRG or possibly for the county or state Department of Veterans Affairs. Just because your time in uniform has ended, your story hasn't.