old gen to M4300 migration - zero downtime

Hmm, I don't think this is going to work without any sort of (albeit very small) downtime if the configuration is corret on both the interconnect between the old and the new stack.

Both switch generations do support distributing LAG members across different switches _when stacked together_. In contrast: The M4500 does multi-chassis LAG (MLAG) - without stacking. Since you cannot stack the two generations together into 1 stack you cannot stretch LAG member ports across these two generations / series of switches.

Thus your LAG trunk between the old and new stack looks sound / sane to me if that is all of your switches that you currently have in that setup.

But I would not attempt having the server connected to both switches at the same time. I'd definitely prefer having a small maintenance window for each server: Assuming  you have LAGs with 2 ports per Server, I would unplug both server ports on the old switch, then plug both into the new stack. Then check if the server and switch bring the LAG / port-channel up and if your server and applications can talk to the servers that are still on the old switch. Then rince and repeat with the rest of the servers.

The issue is, that it might confuse the heck out of either the server or the switches when you try to keep form / bring up a LAG on your server side while the 2 ports are connected to 2 independent stacks that don't know about LAG members between these 2 generations.

This way the LAG goes down from the perspective of the server, this way the server will properly exchange LACP message between the M4300 switches when the ports are reconnected on the new stack.