She is right, MDMA is a horrible drug that will rot your brain and eventually depress you.
Yes, and cigarettes will rot your lungs, caffeine can fuck with your brain and nerves, and alcohol will rot... well, everything. What the fuck is the point of such a statement? Seems a little judgemental IMO, and I was pretty sure that's the shit we stay away from around here. MDMA, when used responsibly, like most other substances, has consequences which can be minimized and negated through preparation and knowledge. I'm not saying it's harmless... no drug is. But it's also no more harmful than other drugs when used responsibly and safely.
To the topic at hand, I'm a little on both sides of the argument. I agree that if you love this girl, then waiting to roll some other time would be no big deal (same with listening to her long, boring stories; as well as not talking about her like you have an extreme amount of disdain for her). However, I also agree that someone who is going on hearsay and ignorant rumors about anything, drug or not, over factual evidence which someone close to them is trying to share, is not a good sign for the future of a relationship either.
Communication is hands-down the single most important thing in maintaining a good relationship, and it sounds like you two fucking suck at communicating with each other, no offense intended. If you really feel all this strongly about her, talk to her about it. Like someone recommended, tell her you're willing to not take it out of respect for her throwing the party, and because you care about her enough to want HER party to go according to her wishes. But also open up to her, and share honest information about it and why exactly you want to do it, and why you think you won't embarrass her. Four ecstasy trips should be enough for you to at least have a bit of a handle on how you behave and carry yourself while on it.
Finally, man, I know you think you're spacing out your E rolls quite a bit, but... you really aren't. The four months space was proper, but only waiting one month between 2 and 3 then one and a half between 3 and 4 is starting to really push it. MDMA, as I said above, can be just fine when used responsibly. However, most peoples' experiences show that frequent use (i.e. more than once in a span of less than two or three months) is absolutely NOT a good thing. At the best, you'll only lose your ability to ever have a decent ecstasy roll again. At the worst, you may genuinely fuck a part of your body or mind up for good. I know it's a fucking blast, trust me, I wish I could just do the shit all the time and dance my ass off. I'd be way skinnier, that's for damn sure haha. But if you plan on remaining a "drug user" (which, nothing wrong with a responsible one IMO), then you should also plan ahead to be able to properly enjoy those drugs for as long as you want to use them enjoyably. Just read some of the posts around these forums of those that have lost the MDMA magic. It's a sad thing to think about.