What makes those called to be 'Evangelists' distinct? Ephesians 4:11 implies that only 'some' are called to be evangelists, but most would agree that we are all to be active in evangelism. If that is true, what makes the former unique? I don't think there is 1 obvious answer, but I do think it's, at least, a combination of the following 3 possibilities.
1. Most of us are indirect evangelists. We are lights wherever we go, but we don't necessarily go to be lights. What I mean is, we have jobs. While we are working we're still Christians and we, hopefully, have opportunity to share the love of Christ. On a part-time basis, we might do some direct evangelism, but we only have so much time to give. Perhaps those called to be evangelists are 'direct' evangelists. The 'go' specifically to be a light. And many of them do this so often that it becomes their full-time job.
2. Most of us, from time to time, notice that our evangelistic juices are running low. We need, every so often, to be re-charged. Perhaps it is the role of the one's called to be evangelists to do that charging. Just like those called to teach are really equipping disciples to teach themselves, maybe Evangelists are really equipping others to evangelize. Their role is to ignite the many.
3. Most of us, despite our best efforts and wishes, don't win tons of souls to Christ. We might move someone from a -8 to a -6, but rarely from a -1 to a +1. Perhaps the calling of an Evangelists is a special gifting from God to be a catalyst for conversion.