Yes, it was difficult to find how to declare client, but i find :
"Client client(4);" out of loop declare a global Client class for server mode, and it work fine : ie you can use client every where in the sketch, i can run my 200ms staff and cut the ethernet request even if the ethernet task have 5s time run.
Note : the line "Client client=server.available();" become "client=server.available();" in the code.
Resolved.