There are a few things here. First, the look at the protocol and see if you can simply determine what number maps to what card. Next, presuming they are doing

There are a few things here. First, the look at the protocol and see if you can simply determine what number maps to what card. Next, presuming they are doing the simple thing (given their choice of random... this isn't that unfathomable) and numbering them sequentially you only have 4 possibilities. Thats not that hard to run your data through. There are a few more orderings that make a certain kind of "straight-forward" sense that would be good tries too. Of course they may not be doing that, and have some sort of "random base deck". That would be a bit harder, im pretty sure you can come up with a system of equations to figure out the card number along with the system described in the article, and as such (and perhaps with a bit more data) still solve it. Finally, there may be statistical methods to combine with the equations in the article to figure out whats happening. (which you may need anyway depending on how exactly get_next_card() is called and how random is called (same prng for the whole system, or one per game? etc)