Check the line speed and duplex settings on the 4 ports in question as well as the end-stations and traffic generator. Neither end should use auto, or default to auto. Explicitly code both the speed and duplex setting on both ends, for all 4 ports. Auto-negotiation does not always work as advertised.
Insure that none of the 4 ports are in the same vlan as the mgmt vlan (vlan 1 by default). The cat switches behave differently when traffic is destined for the mgmt vlan, and you may have run into a case where the degradation is due to the slowdown as the switch may try to examine each frame before forwarding it.
It appears that you are not wanting to SPAN, but instead send traffic out, so you should not need to setup a SPAN port. Just insure that all 4 ports are in the same vlan (though not the mgmt vlan).
If you still need help, then please post the mac-address of each end station, the switch port that each station is on, the switch config and code version (scrub out any sensitive info), as well as a small sample of the traffic you are sending, and I or someone else can try to help.
