Title says a different thing... but anyway: if(strlen(string) > 0) or just if(string[0]) to check an empty string.
About the arguments, you can get all arguments stacked together with read_args() if you're just going to add all of them like that anyway.
And about less than 4 arguments, you should modify cmd_access()'s 4th argument to 1, that sets the required number of arguments, that's why you can't use less.