Try Emacs with vim bindings and you'll have it, its GUI is a client that connects to a server

How would that be better than opening multiple gvim processes?

I want to type :wa in one window, not four windows. I don't want to get an error message when I try to open a buffer that's already open somewhere else, I want it to just move the buffer to the new window (or alternatively, raise the window containing the buffer). I want to move buffers across windows and keep undo history and register contents. TextMate / Emacs / etc. have this capability, but I can't figure out how to do it in vim.