T610 mixing 16GB RDIMMS and 4GB RDIMMS ?

Have a T610, both CPU sockets filled, originally supplied with 8 RDIMMS each of 4GB size (i.e. originally 32GB total).

I now want to add 4 RDIMMS each of 16GB.

If I remove all 8  4GB RDIMMS and insert only 4 new 16DB RDIMMS in slots with white release-levers then all is OK - 64GB and BIOS reports no problems.

But I would like to use as many of the 4GB RDIMMS in the other slots of this T610 if that is possible.

When I tried some mixed 16GB and 4GB RDIMM combinations BIOS 6.3.0 boot  (memory mode = Optimizer) reports 'Unsupported memory configuration DIMM mismatch across slots'. The machine boots OK after pressing F1 to continue and sees 96GB RAM and the operating system boots and works OK, if all remaining slots are filled with the 4GB RDIMMS. I don't want to proceed with 'Unsupported memory configuration' so I revert to 4 16GB RDIMMS for now.

I read the section of the T610 owner's manual about the supported memory configurations but I do not understand it (I don't understand terms like geometry and rank in this context ).

Can I mix the 4GB RDIMMS and 16GB RDIMMS?

If so, how many slots can I use for the 4GB RDIMMS, and which slots can I populate with the 4GB RDIMMS?

Currently the only slots that are populated are A1, A2, B1, B2 (each with a 16GB RDIMM).
The other 8 slots are empty.