As an armchair admiral, I question why they didn't replace the aft twin 130s with VLS or FL3000.
Simplest possible reasons(one and/or combined):
1. Outright counter-productive: If (and when) 956 and 956-EM will be transfered out of fleet destroyer formations(say, replaced with 052Ds) - 130mm guns for coastal formations will nevertheless remain unique to 4 Sovremenniy class DDGs.
Sure, destroyer squadrons will have plenty of them - but they have their own missions too.
2. Undesirable for certain types of missions: Sovs, with their twin separately controlled ak-130s, can very well appear to be the most capable large combatants in confined waters.
3. Not necessary, ship reaches desired levels for self-defence and missile capacity without sacrifices in dual-purpose artillery, and additional money brings better results for an overall capability of PLAN elsewhere.
4. Too expensive to justify for a meagre additional benefit. Ship capability level isn't just about its ammo storage(much like modern subs are about much more than just their average acoustic quietness).
All this against VLS fit. FL-3000.... replace gun with both AA, ASuW and Ground attack functions with quite specific air defense installation is neither right nor efficient if there are no pressing circumstances.
And don't remain weight difference - AK-130 is just many times heavier(in your case, far aft). You'll have to fight lots of ship-associated problems for questionable gain.
Finally, unlike AK-130 itself - adding the second mount to the same sector(in its IR-guided form at least) doesn't necessarily doubles number of targets per time. On the other hand, it's strange to improve one sector(rear) without further updating another.
