I don't think the problem is related to the 4 socket limit.
My reason is that the WebInject test is only using one socket, but I can provoke a failure by using Firefox or IE to send just one more GET.
For example your forms sketch would only generate one GET from IE or Firefox. So the failures must be happening when only 2 sockets are being used.
EDIT : If the socket limit were being exceeded the message would be "Server not responding/busy"
I also checked the max connections per server registry key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\MAIN\FeatureControl\FEATURE_MAXCONNECTIONSPERSERVER
and it is set to 2