phpBB4 is being created from scratch. We want to build a modern phpBB that integrates perfectly with the rest of your software stack. It is going to be built on top of the same popular PHP frameworks and libraries that many other projects use. This will improve interoperability with 3rd party software and make extending phpBB's feature set easier than ever. Useful generic components resulting from our work will be contributed back to those projects.
Regarding plug-ins: Explaining Hooks. And you won't have to wait for phpBB 4.x to get them.
Source: http://www.phpbb.com/community/viewtopi ... 5&start=15
http://blog.phpbb.com/2010/02/18/moving ... velopment/
http://www.phpbb.com/community/viewtopi ... &t=1186015