I selected other.
If you don't want to put two 2-tops together, then pull a third chair up to one of the 2-tops.
The option of asking to share a 4-top hoping the lone person sitting there will take the hint and move sounds PA. Asking the person flat out to interrupt whatever he/she is doing and move to convenience you sounds equally as bad.
I could see myself using a 4-top, instead of a 2-top, because the 2-tops are frequently pretty small for what I need to do and feel very cramped. If there are plenty of 2-tops available, I could see taking a 4-top to use (I don't eat in mall food courts so this wouldn't be happening anyway). because I'd need sufficient room to both set up my meal and set up my laptop/book. I used a bookstand to prop up my book (when I would eat out by myself) and 2-tops were often too small to do this and have what I need for my meal close by. Again, though, I don't eat at mall food courts so I haven't run into the OP's situation.
I agree, also, that you don't know if the solo diner is waiting for others. I was once at a table with four chairs at the fair (and available tables were at a premium). Some guy came up to my table and started to take a couple of the chairs without asking or acknowledging me. I stopped him immediately. Why? Because I'd been sitting there holding the table while my friend went to get our lunch. The other chairs were for him. I thought it quite rude the guy didn't even bother asking me before trying to take them. You might think two chairs for each of us is special, but I'm really short. Frequently, chairs are too high off the ground for me to sit comfortably because my feet don't rest flat on the ground and the seats are too deep for me rest my back against them and still be able to reach the table comfortably. I think I used the lower rung of the chair to rest my feet against, which gave me the support I needed to sit upright.