Share-Based Payments - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2020	Jan. 31, 2019	Apr. 30, 2019
Share-based Payment Arrangement [Abstract]
Stock-based compensation expense	$ 229,000	$ 335,000	$ 437,000	$ 498,000
Option modification charge (reduction)				$ 7,500
Weighted-average grant date fair value (in usd per share)	$ 2.56	$ 5.76	$ 3.09	$ 6.03
Warrants outstanding (in shares)	1,669,773		1,669,773		1,671,440