Borstock said: This is a game. You like Carol? Playing with her is fun for you? Then you made exactly the right decision.
I'm going to help.
I'm a 5* player and I cover "latest" 4*s quickly. When vaulting was in full effect it was about 4-8 weeks. I think it's about twice the time now, but anyway, I ended up with 2/5/5 4* Rocket and Groot. After letting two blue covers die on the vine, I drew a bunch more blue and green and when those were close to expiring I said screw it and just paid the 120 CP to buy that last yellow. I don't regret it for a split second - that's 4 "latest" or 6 "Classic" pulls and only had a 1/3 chance of drawing the color I needed any time I got R&G.
In your case you're a 4* player and you needed two of the same cover, and the character was about to leave "latest." Even if you set her as your sole 4* Bonus Hero, the chance of you covering her in a timely fashion was really, really low. Not only that, but she will definitely directly help you in the game - I guarantee you will play her in both PVP and PVE when she's boosted.
That 240 CP represents 12 classic pulls (or 9 latest pulls, but you probably wouldn't do that.) Those 12 pulls, on average, would be 1.8 5* (no immediate help for your roster, really,) 5.1 latest, 5.1 vintage. Unless you got some very specific covers, they would not have been any short-term help to your roster. Would they be long-term help? Sure, but quite probably not as much as getting a Champ Captain Marvel weeks or even months earlier than you would have. Also, you got another pull from champing CM.
I agree with everyone who is saying that you made the right call because it's the most fun thing to do. I'm also saying I would have done the exact same thing in your situation.