Well, if you're replacing the card each time, then each time it's going to be a 13/52 every time. If you aren't replacing them, then you subtract one for each for trials, like so:
1st trial: 13/52
2nd trial: 12/51
3rd trial: 11/50
4th trial: 10/49
That's for each trial individually. Together, I'd say that's 48 possibilities.
