Note 14 - Concentration (Details Textual) - USD ($)	3 Months Ended		9 Months Ended
	Jan. 31, 2021	Jan. 31, 2020	Jan. 31, 2021	Jan. 31, 2020
Revenue from Contract with Customer, Including Assessed Tax	$ 10,130,000	$ 10,803,000	$ 30,668,000	$ 30,500,000
Customer Concentration Risk [Member] | Revenue Benchmark [Member] | One Single Customer [Member]
Concentration Risk, Percentage			32.10%
Revenue from Contract with Customer, Including Assessed Tax			$ 30,668,000