Can i run a minecraft server on a cluster of 4 or 8 raspberry pis (model B+), and use all core and ram for the minecraft server?
With some reservations, the answer is probably "No".
That is not to say that you can't make a cluster of raspberry pis (because you can), and not to say that you can't run minecraft on such a cluster (because you can), but rather that currently there is no implementation of a minecraft server that would actually benefit from such a setup. That is - the performance wouldn't increase.
What minecraft needs is relatively few cores that run on very high speed and a lot of common memory.
I run a minecraft server on hardware built for an htpc. Such a setup can probably be comparable in pricing to an 8 node raspberry pi-cluster, and work a lot better for this usecase.