Certain design ideas and indeed some code have been borrowed from JBoss Cache 3.x, however JBoss Cache is in no way a dependency. Infinispan is a complete, separate and standalone project. Some may consider this a fork, but the people behind Infinispan and JBoss Cache see it as an evolution, since all future effort will be on Infinispan and not JBoss Cache.
The current release of JBoss Cache, 3.2.3, is considered stable enough. No new features will be added to JBoss Cache or POJO Cache, although critical fixes will still be addressed. Consider both JBoss Cache and POJO Cache to be in maintenance mode.
So, why not just call it JBoss Cache 4?. Several reasons. Binary and even API compatibility with older JBoss Cache releases is no longer maintained, and the new API is simpler and easier to use. Also, the internal data structure used is not compatible with JBoss Cache.