They can justify the replacement of the 3400g with the 4300g by noting that the CPU cores are considerably faster than the ones on 3400g. Yes, the gpu section is a slight regression, however, we haven't seen what its capable of with decent RAM and a decent overclock. The scores aren't night and day different at stock for GPU performance, and we know that 3200/3400g had issues with pushing RAM much past 3200-3400 speeds. Early benchmarks are showing that Renoir can push well past 4000, which can make up for a lot of the memory bandwidth limitations of the 3200/3400g. I see no reason to be disappointed at present and believe that most 4300gs will make it to a 1900mhz clock speed and support high DDR4 speeds and will be, effectively, similar to the outgoing 3400g in actual usage.
I base a lot of this on the comparison between two of my kid's laptops. I have one with a 3500u laptop and another with a 4700u one, both with similar TPU limits. While the 3500u has an additional CU over the 4700, and both have 8 threads, the 4700u is comfortably better than the 3500u in every game that we've tested so far. Now we can point at the 4700 having 8 real cores instead of 4/8 like the 3500, but, in graphics power, the raw theoretical throughput numbers aren't much different between them. However, the RAM on the 4700 is faster, and it has much better CPU performance. Both of those should be the case in comparing the 3400g to the 4300g.