I recently noticed that Pi 4 integrated NIC caused a lot of troubles (first packets loss then following ones transmitted fine) compared to others NIC.
The issue is due to "EEE", some green washing technology used to save some milliwatts, decreasing the stability of the link. Most of the time, "EEE" works fine however on all my Pi 4 it's causing troubles to me.
It can be disabled by playing
ethtool --set-eee eth0 eee off - on OpenWrt, just add it to /etc/rc.local (so it's disabled at every boot).
On my "ONT" (GPON converter fiber <-> ethernet) it's disabled by default, which makes me think I'm not the only one to think this should be disabled on things like routers.
ethtool --show-eee eth0 can be used to see whether it's active or not (and to check if disabling was successful)
It's a Pi 4 problem so I created a thread on their forum https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=305820 but as it's also affecting OpenWrt, so I believe it's useful to let people know about the issue. Not sure if this issue can be corrected by firmware or if it's an hardware poor performance of the Pi 4.