I bought one just like yours yesterday from bestbuy (Q304UA-BHI5T11) however it too reads Q304UAK on the DIMM slot. I don't know if the published max of 8GB for the slot is correct or if you can go higher with FW updates, however I can confirm Crucial is wrong about installing 2 DDR4 2133MHz DIMMs. I opened the back cover and there is one 4GB memory slot built-in in to the MB, and a single slot with a 2GB DIMM installed (replaceable). If someone determines that you can add a DIMM larger than 8GB to the slot, and the net memory exceeds 12GB recognized by the OS, please comment here with the size of the add-on DIMM. Thanks.
Here is the URL to the product on ASUS's website. Notice it says DDR3, which is definitely an old spec since it is actually DDR4.