2 versions of datanodes currently live

[#1]

Is it ok to have 2 versions of Hadoop in same 4 nodes cluster?

Here's what I have in the panel:

There are 2 versions of datanodes currently live:3.1.1 (3) ,3.1.3 (1)

Should I upgrade 3 nodes to 3.1.3 or downgrade one node to 3.1.1?

[#2]

The DataNodes should run the same software version as the NameNode.

[#3]

Any details, explanation or pointers on why this is the case?