MySQL cluster DataMemory and DiskPageBufferMemory parameters
DISCLAIMER : Not SCMCDBA Certified
If you plan to hold data in memory, you need to

increase RAM on all data nodes to 16GB
increase the DataMemory to 6GB
increase the number of SQL nodes to 3 (Makes SQLNodes to DataNodes 1:1)

If you cannot make these changes, then you should offload data to local storage

MySQL Cluster -- Storing table data on data nodes

Here is one of the best answers from Frazer Clement : MySQL Cluster RAM requirement
He states in his answer

When configuring DataMemory and IndexMemory, it is best to limit yourself to somewhat less than the physical memory in each system. Some physical memory should be reserved for the OS and other processes.

There are three MySQL Cluster Gurus in this forum

Frazer Clement
Mat Keep
Andrew Morgan

Checks out their answers !!!