4x4 means having a 4-antenna array and can simultaneously connect to 4 MIMO streams. The advertised maximum speed of your Wi-Fi card requires that the router you are connecting to also suppports 4x4 MU-MIMO and they are communicating on an idle 5GHz channel by themselves using the 802.11ac standard.
NitroQAM is just 1024-QAM under a brand name. It's a non-standard implementation in 802.11ac, but standard in 802.11ax (Wi-Fi 6). QAM is a family of modulation methods. 1024 means the carrier can have 1024 states (a.k.a symbols), thus each symbol can carry 10 bits of data (log2(1024) = 10). In QAM, carrier states are conveyed by 2 sinusoidal waves (customarily called I and Q waves) with a 90-degree phase difference (orthogonal). The varying combination of I and Q waves's amplitude and phase represents different states. Constellation diagram is often used to visualize QAM.
When both the router and Wi-Fi client support 1024-QAM, each symbol can carry 25% more data compared to the standard 256-QAM (log2(256) = 8, 10/8 = 1.25, so 25% more). If their symbol rates are the same, that equals 25% more data rate.
Basically, Asus let you use some 802.11ax features on 802.11ac. In order to utilize them, you need a 802.11ac Wave 2 router since MU-MIMO is an optional feature. NitroQAM, on the other hand, is completely proprietary, so good luck on that.
If you ask me, just be happy or use ethernet cables.
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