Hey, this is only true for your specific CPU & MoBo combi but not as a general statement.
According to your MoBo Manual, once you use 2 PCIe slots the bandwidth from the first slot drops from x16 to x8. And I suppose the ASUS card is using 4 PCIe lanes per SSD therefor with 8 lines you can only use 2 SSDs.
However if the first PCIe slot drops to x8 depends on the CPU generation and chipset, lots of the newer boards don't have this issue, Server Motherboards usually have more PCIe lanes then you will ever need and even some older platform don't have this problem neither.
So yes in your case it is unfortunately not possible to use the full 4 SSDs from the card in combination with a HBA card (due to HW limitations) , but for other setups this might not be a problem.