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

You may feel good now but those with no previous injuries, our bodies do not get better in the army life as we get older, we break down. Take your success now and run with it (not to a recruiter). You got to play with the 82nd, jump out of Army planes, and have more to be proud of than 99% of society. It sounds that if you want to be walking in your sixties you need to accept this outcome. Spinal injuries can effect your sex life and relationships, and are not something to think you are close to 100%. You did more cool shit in the Army than I did and moving on is probably best for you.

RE Code 4: Normally not eligible to reenlist, nor join another service. Usually will require an Exception to Policy waiver to reenlist. (Google, Internet) Possible, but depends on needs of Service vs. your qualifications.

Get yourself an Army Civilian job if you still feel the need to be a part of the Green Machine.

You need to speak to a recruiter....but dollars to pesos it ain't happening

You're RE4. That pretty much means you are a no go. It does not matter about Honorable discharge. You are medically retired, you will likely not even pass muster to make it to MEPS. But miracles do happen. You can always try a recruiter.

you are retired, period

You can always check with a recruiter but I would be seriously doubtful it will work. Med Boards resulting in medical retirement are generally pretty final by design.
This is what you can find online about RE4

RE-4: Individuals separated from last period of service with a nonwaiverable disqualification (refer to AR 601-280). Ineligible for enlistment except as provided for in paragraphs 2-7c and 2-7d. (See waiverable moral and administrative disqualification.) Disqualification is nonwaivable.
RE-4A: Individuals who did not meet basic eligibility citizenship requirement of Chapter 2, AR 601-280, at time of last separation from active duty. Ineligible for enlistment unless requirements of table 2-1 can be met. Citizenship requirements are nonwaiverable. Applicable to EM who fail to meet citizenship requirements.
RE-4R: Individuals retiring after 20 or more years active Federal service (title 10, U.S. Code 3914 or 3917) Ineligible for enlistment.

It's possible to get a waiver, you'll have to talk to a recruiter. You do need to ask, are you good to go or still going through therapy? Additionally, depending on how long you've been out they might make you go back through BCT again. So make sure to ask about that.

If this is good for you, then go for it. I can tell you 99% of Guard physical readiness is on your own. So only you can answer if you're ready to get back into training, and getting back into uniform.

So dig deep and speak with a recruiter about the process. If you decide to go another route, America still needs people to serve in other capacities.