Note 3 - Accounts Receivable, Net, Contract Asset and Contract Liability (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2021	Apr. 30, 2020
Accounts Receivable, after Allowance for Credit Loss, Total	$ 1,961	$ 2,784
Accounts Receivable, Allowance for Credit Loss, Ending Balance	143	143
Contract with Customer, Asset, after Allowance for Credit Loss, Total	421	195
Increase (Decrease) in Contract with Customer, Asset	226	(286)
Increase (Decrease) in Contract with Customer, Liability	3,609	(1,050)
Contract with Customer, Liability, Revenue Recognized	$ 2,100	$ 2,700