There is a chace that the deca was causing it. Everyone responds differently to hormones and you may be more susceptible to progesterone related acne. That would be the most likely reason. There is also a chance that your test levels have begun to stabilize at 4 wks in, where as before they were consistently moving up. Flux in hormone is, from what I've seen and experience, the most common reason for acne. Keep an eye on yourself, at the end of the cycle, as you come off the test. There is a chance that acne could flare back up, as your levels begin to drop