dnpp123, from what I can tell just by looking, I'd be willing to bet that board works good. And the extra fan by the PCI connector is a nice bonus. But please don't take my word for it, I only just put in a pre-order on the next batch for a S4.
The max TDP for the L9i/a, as far as I have read, is 84w, and this is currently the best performer for the S4 according to Josh's analysis. So I guess it just depends what you are doing. If gaming is your main thing, you "technically" don't need more than 4 cores, or 6 without hyperthreading. If you want to use a beast like the 9900K or upcoming 3950X, undervolting, and even underclocking will likely be mandatory. The reason you see those higher TDP CPU's is because only the K-scu models allow for making the necessary adjustments-- specifically on Intel processors. You might have heard Josh mention how some builders were able to get underneath 50w TDP, which is impressive. This is just based on my limited understanding. But hey if you're into it "just because," absolutely go for it.
*AFAIK* PCIe 4 is 100% backwards compatible-- on the motherboard. Expansion cards using PCIe 4 will not be compatible with a PCIe 3 slot. *AFAIK* I suspect the connector will be a little longer physically, but it's strictly throughput, there's no computation being done here. You will not gain any performance benefits using a standard desktop graphics card UNLESS you are getting up to like, dual 2080's and beyond. But theoretically you wouldn't even need any kind of bridge for it. Thing is, that isn't really the main benefit to the new standard, what I'm seeing says it will mainly benefit PCI slotted hard drives, as currently NVMe borrows some lanes for it, and even robs performance if you hook up a second one. Again, not even useful for gaming at present, because you don't even bottleneck on a SATA interface SSD.
But yeah, it's a super exciting time to be a PC builder. 
