At the end of Obama's fiscal year (Sept. 2017), it was 4.2.
Trump inherited a decreasing trend from Obama and it went on under Trump. It's not like the trend was flat and suddenly decreased under Trump. Trump was surfing on Obama's waves from his last mandate, doing a bit better on growth, a bit worse on inflation, a bit worse on debt, and a bit worse on unemployment reduction rate (which as you noticed, is not unusual, curves usually get more flat as they approach the minimum or maximum values).