<p>We're starting a large web project, mostly green field. I like the Tapestry framework for java/web solutions. I have concerns about starting a Tapestry 5 project since T5 is still in beta. However, if I understand the documentation correctly, T4 development will not be supported by T5 and up. My question: Should I begin a large project for a large company with T5? If not, with the imminent release of T5, should I ignore T4 altogether?</p>

T5 is in the last beta, the next release is RC, and then the full release. according to howard, things should be done by the end of october. so, if you're starting the project, i believe the framework will catch you with its release.
another thing, the betas are pretty quality products, howard does the great job with his framework. at my now previous company, there is a project started in september based on tapestry 5, a colleague managed to get a working example pretty quickly and it seems okay. we worked with tapestry 4 on a previous project, and when the question raised about which version to choose, the fact that T4 will be abandoned in favor of T5, and much changes in the framework concept itself, the counclusion was it's much better for a developer new in tapestry to learn new version immidiately (also, if i were to stay, i was interested in switching to T5 also as soon as possible, because i see a quality improvement in the T5 concepts compared to T4 which i worked with).
of course, you have your deadlines and project limitations which you have to take into account. if it's rather flexibile, or long-lasting project, maybe get a quick start of T5 for a week, and then decide based on your experience with it.