it's (eq arg 4) to be compatible with `nrepl-switch-to-relevant-buffer`. 

Since nrepl-swtich-to-relevant-buffer always calls nrepl-switch-to-repl-buffer with an argument, we need to check the actual value of the argument in this condition. It didn't matter earlier since `nrepl-switch-to-repl-buffer` was (interactive "P") - the default arg would be empty list which was equivalent to nil.