I have the strong suspect that - actually - Aruba 5400R zl2 Switch can't split a single QSFP+ 40 Gbps interface into four 10 Gbps sub-interfaces as it happens on some Comware operating system based Switches.
On Comware...splitting is an operation that is related both to logical level and to physical level (configuration at logical level enables QSFP+ ports to be splitted into SFP+ sub-interfaces and this involves that connected Breakout DAC Cables can be recongnized): see what happened on QSFP+ physical interface where the usage of the using CLI Command let the Switch to split the FortyGiga port into four sub-TenGiga ports and that operation enabled the breakout DAC cable to be seen finally, read this post.
Moreover I don't see - on latest Aruba 5400R zl2 QuickSpecs (here) - any reference, as one would expect, to supported breakout cables (from 1xQSFP+ to 4xSFP+)...and, I fear, this lack of reference could be a sort of dead end road if you're planning to use QSFP+ breakout into 4x10Gbps ports.
There was also a discussion on reddit few months ago about Aruba 3810 and QSFP+ breakout into 4 SFP+ (read about it here)...Aruba 3810 isn't like the Aruba 5400R zl2 with the J9992A Module...but I think that was quite indicative.
Plase correct me if I'm wrong....would be interesting to discover that ArubaOS-Switch recongnizes automatically a Breakout cable and enables four TenGiga sub-interfaces on the main FortyGiga interface...without user intervention.