I've had a chance to play around with a few 3400+ CPUs now. On an MSI board the BIOS does not have an AUTO setting and defaults to 5x, but it boots and runs just fine. On an ECS board AUTO and 4x works fine, manually setting to 5x causes no POST.
So far I've tested three out of the four 3400+ CPUs currently in my possession and the best overclocker does around 2.6GHz somewhat stable. By "somewhat" I mean that it seems to run okay, but will eventually (but not immediately) fail stability tests like Prime. That is actually at full default vCore, so likely it will be stable with a slight boost of vCore, or a slight drop in HTT.