Can I decline being promoted to Corporal?

A little off topic but dang I wish I was you , promoted to Cpl and Ranger school , You joined the army son be all you can be . In your old age your time in the military will be the highlight don't leave nothing on the table .

Read up on declining promotions. My first seargant once looked a counseling on a soldier saying he didn't want to get promoted. Declining a promotions may get you barred from renlistment

Everyone has already gotten the point across. You realize by turning this down you're giving the middle finger to your CoC and there are consequences to that. You realize they don't have to send you to ranger school and turning this down will seriously destroy your chances of going because they will give it to someone else more interested in leading soldiers. We all had to do jobs above our pay grades, life sucks drive on. It's about the soldiers you lead, teach, and mentor not a paycheck

SGT (Join to see) Your choice to deny an opportunity will come with consequences. Think through the effect your action will have on those who have, up to that point, considered you a potential leader. To hear, what is in effect, "not my MOS", will cause some serious rethinking.
Your question sounds like a union member who is being asked to perform a task outside the specific terms of a contract. Not the question from an outstanding military combat arms troop being provided an opportunity to demonstrate capability.
I have been away from active military life for many years, but still remember accepting any chance to learn and develop when it was presented.

SPC Josh Harrison, do yourself and the military a favor and decline the promotion, decline the promotable status, finish your contract, and get out.
Your “Why should I take the responsibility if I don’t get the pay?” attitude is part of what leads to the poor leadership that many complain about. Would you not lead the remainder of a convoy out of an ambush fire sack because you aren’t going to get paid the same as the OIC or NCOIC that got taken out? Or maybe not do your best to learn CLS skills because you’re not a medic? Many times, in both civilian and military life, you are going to learn that you have to do more than what you get paid to do.
Personally, I think the Army needs to follow the Marine Corps lead and abolish the SPC rank, period. Last “guaranteed” promotion is E-3. After that earn those stripes.

I don't believe you can. I want to say this is the new way forward for the Army. It came out in a MILPER message I believe. But instead of getting a "P" status the Army will be making them CPLs instead to start the mentoring process as a leader. I think this is a great idea. It is a challenge but then so is leadership. If you don't want to accept the lateral promotion you can request to not be promoted and rescind the board results. In all honestly I believe the intent is to leave you in place as a CPL in your current position. Your unit may try to place you into a leadership role in preparation for when you come back and are promoted to SGT.

unless I'm mistaken or out of date the military pay chart shows E-4 no E-4 + or -, just E-4, money is the same, responsibility as a team leader can and often times does happen whether you are a Spec or corporal unit dependent, your refusal because of money is weak at best, and shows why your schools BLC and Ranger should be pulled, if your not willing to take responsibility above your pay grade get out period quit wasting the army's time.

The OP is being fast tracked, probably made Specialist with a waiver, and went to the board in his secondary zone - He is way ahead of his peers. If he succeeds as a first time go at Ranger School, and doesn't get peered out, he will likely make SFC by his 10 year mark. 1SG by 12, and either CSM or SGM by 15. This Soldier, with a Ranger Tab, will walk into the Staff Sergeant board, recite a paragraph, or line from the Ranger Creed, and walk out Promotable. This troop is being set up for success, and being rewarded and recognized, He has a coveted Ranger School slot, has already been to and passed RASP, will absolutely get Airborne school, and also any school of his choice in his MOS for further advancement. In return, he is being asked, as an E-4, to showcase his skills and abilities, which is exactly what a Specialist in the Infantry should be doing. I do not see the Army abusing this Soldier, I see the army putting a spotlight on him as a rising star.

The only thing that will halt this process is the Soldier himself, with his attitude. Trust me to know how my MOS works, and what is happening with this particular Soldier is by no means unfair treatment, nor leadership looking to exploit him. He is being rewarded in the highest form of praise the infantry can apply. He is going to lead a Combat Fire Team.

If you pin CPL you will be an NCO I served as CPL in the USMC and a SPC in the ARMY and wished they would have made me a CPL and the extra responsibility that went with the RANK , CPL’S LEAD SPC follow

You can talk with your leadership about it.