Hi Rafael,
Merging two clusters is not supported. If you have sufficient capacity on the four node target cluster, you can snapmirror the data from the source two node cluster to the target one.
When the snapmirror is finished and the data being served successfully from the target cluster, you can wipe the source cluster and join it as new nodes with new aggregates onto the four node cluster, if the platform mixing rules allow it - some of our lower end or older platforms have cluster size limitations which may prevent this.
Regards,
Alex