You may see a discrepancy in the stroke rate and split metrics shown on the Ergatta app and your S4 monitor. This is expected due to a difference in the time frame over which these metrics are calculated. The Ergatta app uses a more real-time calculation of these metrics to make it feel more responsive when you speed up to catch the opponent ahead of you, or when you adjust your cadence to hit new targets. The SPM that you see in the Ergatta app is more reflective of your real and current SPM output, whereas the S4 calculates SPM as a rolling average over 7 strokes. Over long periods at constant speed and stroke rates, you should see the Ergatta and the S4 converge on the same stats.
You can learn more about how Ergatta calculates SPM HERE.