4-FA and ecstasy/MDMA
Some people use MDMA when they notice that the effects of the 4-FA are fading away. By combining they prolong the euphoric high. Redosing 4-FA just ensures that you stay awake longer. It hardly increases the euphoria. That is why MDMA is chosen. If you choose to combine, take less of both drugs than if you were taking only 1 drug.
Both are stimulants. The combination increases the load on the heart and blood vessels. 4-FA has an unpredictable effect on blood pressure in some people. It can get way too high. In combination with MDMA, this blood pressure can become even higher.
Combining MDMA and speed increases the risk of brain damage. This is probably also the case with 4-FA and MDMA.