I don't know that there's really a quicker solution.
The easy solution is to take it to a body shop and have them re-spray it. But with ease comes cost.
Think of it this way ... the paint has been neglected for 4 years. You're putting in 4 years of work all at once. It's worth it to take the extra effort, because you incrementally make progress instead of trying to guess how much is needed.
In your case, you're taking down an elephant. You have an assortment of weapons at your disposal, but know that choosing the wrong weapon can have negative consequences. Let's call XMT1 a BB gun. You could use XMT1, but it's going to take a long time, just like it would take many many shots to kill the elephant, or you might never get there. Maybe you take a .22 instead ... it might take a while, but you know you'll eventually get the job done. Like knocking it down with XMT4. It might take a while, but XMT4 will EVENTUALLY get the job done. Or you can jump straight to the RPG and obliterate the elephant, but you know full well that there is going to be collateral damage. That's the equivalent to jumping to an abrasive that is too heavy.
If you get something too aggressive, you'll do irreversible damage, but slowly working your way down with something like XMT4 might be painstaking, but you'll at least know when to stop.