Would a letter of recommendation from my former Commanding Officer (who separated me) help me to reenlist with an RE-4?

Follow the advice of the Gunney in this thread. The National Guard can waive just about anything besides felonies.

I would talk to someone who is a great admin to get that waiver going and advocate on your behalf. I’ve learned there’s a waiver for nearly everything just don’t give up or accept every “no” as an answer.

I'm afraid not, my friend. Your best route is to try and get the code upgraded.

Your can what ever you want from whomever you want. When you got to re-enlist or join another service no one can change or avoid that code. They will gave to deal with it. The letter from your Commander can be used in the waiver.

I'm going to say 'no'. Generally, anyway. RE-4 means not eligible for reenlistment. Otherwise, I'd be back in the Army now. If the military is in dire need of bodies to replenish the force, maybe they'll issue a waiver. But I doubt it.