This will probably depend on Cactus.
At Javapolis last week, I had the opportunity to sit down with Petar Tahchiev, who is reviving the Cactus project. One of the things he talked about was support for JUnit 4 and TestNG. He's planning to do a release in the next couple of months but I don't remember if JUnit 4 was targeted for that release or not. I'm pretty sure it's on the road map though.
To anyone who is interested, I'm sure that Petar would love to have some help on this. If you can't find his email and you want to help out, just send me a note and I'll put you in touch.