Likely that's where the multicast traffic, and perhaps all traffic, is queuing and also overflowing the queue.
Since you mention the traffic is mainly multicast, there might not be a real advantage to try adjusting queues.
Perhaps the easiest thing to try is to increase the bandwidth between switches and or change their topology.
You mention "Connected sequentially by 100M Ethernet copper", if that's literally true, you might define one switch as the "root" and connect each of the three others to it.
You might also consider, if not used already, using your gig SFP ports with copper. Depending on model, you might also do this too in a star configuration.
You might consider stacking the units, if they're physically close enough.
You might consider defining an EtherChannel with two ports between switches (if you do this, check whether the default hash algorithm is acceptable, if not, change it.)
