While serotonin syndrome is unlikely, even on the SSRI, my understanding (which isn't great, I'm just giving what advice I can and hope someone who knows more chimes in), is that you're only just adjusting to a drug that prevents serotonin levels from dropping, then discontinuing it abruptly in favour of a drug that causes serotonin levels to rise.
Without knowing what rebound or returning symptoms you might get, if any, from stopping sertraline at such an early stage and without knowing why you started taking it, it's just a guessing game of how it might go or what side-effects/after-effects you might experience.
I would suggest either giving the sertaline time to work and skipping the MDMA or speaking with your doctor about discontinuing.
I have been around people who just stopped taking their antidepressants suddenly and it's always been upsetting, sometimes a bit frightening to be around IME. Granted, they'd been taking them longer than four days, but it's not really advisable to just stop and start.