Yup yup yup. The game wasn't set up to have four players so it wouldn't lag (although it does help in that regard). It was set up that way because it makes for the maximum amount of realism and balance.
I understand how much more complex SCPT is in terms of what needs to go over the network, but on my DSL I've been connected to 56k servers and not felt any lag at all. SC is built off the UT2003 engine after all, they just added lighting, AI, and new controls. I would imagine they kept the UT netcode, just modified to use their engine tweaks. As long as they're not hosting from the backwaters of Japan, a 56k should still give decent pings provided the clients have good connections.