Esther, it is likely because the channel mapping is not set in the timing device setup. Click on Local Configuration, Timing Device 1, set to the S4. In the lower part of that screen is the channel mapping, the tab labeled Time Measurement. Click the drop down menu and set C0 for start and C1 for finish. You can use other channels for intermediates, following the same procedure.
Click Test Timing Device at the top. If satisfied, then SAVE the configuration at the bottom of that screen. I suggest naming it S4 configuration or something that makes the most sense to you.
Remember, the timers are synchronized to each other and the software is only asking for the time as a reference.