You can join any server as long as you have the access to the map currently playing, if the next map is Premium content that you do not have then you will be kicked.In BF3, you can designate servers as premium and presumably, if you do not have the premium pack, you can not join.
In BF4, I only bought the base game but have not premium. Does anyone know if they will have "premium" servers from day one? And if so, will I be able to join them (I most certainly have all the maps through the first expansion).
Everyone will be "base players" until the first Premium DLC releases.