I've been with this company for 4 years and was just recently "promoted". It's really a lateral movement with a better title and more money. I do not feel I would be able to stay in this role for more than a year.
That being said I was just offered an opportunity where I do feel I would be able to learn and grow. I am wondering if it is unethical to resign so soon after receiving a promotion? Also, any tips on how to resign?
I wouldn't say there's any issue leaving. You've been there four years which is a fairly long time nowadays and if you dont think you'll be happy in your new role you should move on, especially since like the change was done without your employer discussing it with you. Normally such a conversation takes place specifically to ensure the employee is comfortable with it, and your career path should have been important to them.
Just resign like you normally would; you appreciate what they've done for you so far but you're interested in the new opportunity.