While using the lastest dev version of forestry I got the following crash
http://pastebin.com/hcVacum0

Forestry 4 has been pushed to stable, this has been fixed.

It looks like they renamed something internaly

Forestry underwent a fairly large API refactoring a little while ago. This may be the same thing that broke Biomes o' Plenty's integration. Until it's fixed, Forestry #639 will work.

I wont fix this, until it breaks on the stable branch of forestry.