AT&T Unlimited 4 Smartphone Requirement

My family has 3 smartphones, all on grandfathered unlimited data. We also have DirecTV, so it would save me some money to switch to the new unlimited plan they are offering. I tried to change the plan tonight on the website and it said I must have at least 4 smartphones. I know the 4th line is technically free after a monthly rebate, but I don't have a phone and am not going to buy one just for this, unless it's some kind of cheap GoPhone. Do you think I can go into the AT&T store tomorrow and actually register another line, but without a phone, and have them just give me the sim for the 4th line? I would think they can assign a phone number to that sim and I'll just store it some place until I run across a cheap phone to put it in and use it as an spare. I'm just wondering if they'll give me a programmed sim without an actual phone present to put it in.