What is considered low or high frequency?
It depends on the application. In this case, it's relative to the frequency of the time base.

As the period of the measured signal is not necessarily a multiple of that of the time base, the ticks counted are only an approximation for the frequency and there will be some error in your measurement. However, the more ticks fit into one period of the measured signal (the higher the frequency difference is between the two signals), the smaller the error.

Knowing the period of your internal time base and the desired precision, you can decide whether the signal is low or high frequency for your measurement.

For example, 4100 Hz seems like low using a 10 MHz time base (2439 or 2440 ticks per period, depends on the starting phase difference). The error can be calculated: as the measuring device can only count integer ticks, exactly 2439 ticks would mean a frequency of 4100.04100... Hz, while exactly 2440 ticks would account for 4098.3607 Hz. In the first case, you have an error of 0.041 Hz (0.001%), in the second case, the error is 1.6393 Hz (0.04%).