what does it mean when a person is arrested for 4 counts of robbery?

It means there are four separate law breaking points that a person is being charged with, rather than just one.

For instance, breaking and entering is one count, destruction of property can be another, actually taking something away is another...depends what all happened during the incident. Or if there were four separate people who were robbed by the individual, that's four separate charges of robbery, one for each person robbed.

It can mean that they are accused of 4 separate incidents of robbery; but it can also mean that they are accused of one incident of robbery but that they are accused of breaking 4 different laws in that one incident.

If you rob four different banks, that is four counts. However if you rob one bank and three sustomers in the banks, that is also four counts. It just means four occurrences. You can not tell exactly what without the details.