First, welcome. Second, if you're going to use either the LCD-X or LCD-MX4 for tracking where you're near a mic (like a vocalist or strings player) because you will get some sound leak. Lastly, the LCDX21 is better tuned than the prior years, but the MX4 has better technicalities, detail reproduction, and dynamics - if this helps,
here's Crinacle's FR comparison graph of the two and you can see the bass is better extended, there is a little deeper dip in 7k and 9k in the MX4.
I'd be super interested in a MX4 2022 because even greater weight savings is appealing, until that exists, I'd still take the MX4 (B-stock price) over the LCD-X-21 (used/new price).