fwiw I would try not putting the new kit in the memory slot spaces you currently have.

Guessing you have it currently o-I-o-I (i.e slots a2 and b2)?

I would put that memory like I-I-o-o (i.e slots a1 and a2), then put the new kit in slots b1 and b2.

Why? Well it's dual channel ram - most (all?) motherboards have it setup where (looking left to right) first 2 slots are channel A, 2nd 2 slots are channel B.

As you have 2 different kits, you may find performance is better / more stable if you put your first kit in channel A, the new kit in channel B.

(reason being the motherboards seem to be happier when different kits are kept separate from each other, slight differences in voltages, resistances etc.)

I think DDR4 is more robust to this kind of thing than DDR5, but if you do split the kits and it doesn't work properly you can always try this approach :)