Another thing to watch out for is that qemu only listens on 10.0.2.4 port 139. I just tried smbclient on feisty and it seems to try port 445 first, by default, and that ends up going straight to the host smbd rather than the qemu-controlled phony one. Try "smbclient -p 139 //10.0.2.4/qemu" and see if that helps.
Another thing to watch out for is that qemu only listens on 10.0.2.4 port 139. I just tried smbclient on feisty and it seems to try port 445 first, by default, and that ends up going straight to the host smbd rather than the qemu-controlled phony one. Try "smbclient -p 139 //10.0.2.4/qemu" and see if that helps.