I think the pins on the TRN are the same. I think they list 0.75 and 0.78 to sell upgrade cables to the KZ folks that need 0.75 and to the other folks who have 0.78 IEMs (more of the industry standard size). Just pick one and go for it. They are basically interchangeable.
As far as the ZS4, it is definitely better than the ZS3.
As far as having better sound than the ZSR, that’s kinda a toss up. The ZSR has better midrange. But it fits bulky and has a huge nozzle. The ZS4 definitely fits better (for most people). And IMO ZS4 has better bass and better isolation, but at the expense of the soundstage. I like the ZSRs soundstage.
If I had to rank them, given not just the sound but also the value/price ratio and fit, I would go:
1. ZS4
2. ZSR
3. ZS3
But 1 vs 2 is very close; almost a tie. The ZS4 would edge out the ZSR due to the better price, better fit, and more isolation. But ZS4 is more v-shaped than ZSR, so if you want a more shallow v-shape stick with the ZSR.
I don’t have the ZSN yet, so it’s possible it’s better than all 3.
KZ has so many models now, that many of them are just side grades. They all have good build quality, good fit, good accessories, and better than average sound. However, most KZs sound similar with minor nuanced differences between them.
A lot of it is splitting hairs. Like the difference between a vanilla cake with dark chocolate frosting vs a vanilla cake with milk chocolate frosting. Sure, the cakes are different, but not really that different.
I’m sure there’s a “big difference” between the EDR1 and the AS10, but it’s 20xs the price. But there’s not a “big enough difference” between the ZS5 v2 vs ZS6, the ZS4 vs ZSR, the EDR1 vs EDSE, ATE vs ATR, etc.
Bottom line, is if you have the ZSR, and you’re perfectly happy with it and there’s nothing wrong with it (sound, fitment, it’s not flaking out, etc), I’d pass on the ZS4. Maybe instead spend the same $12 you would have spent on the ZS4, and put it towards a Bluetooth cable, or a wired upgrade cable, some better tips, a couple of nice carry cases, etc.